Descriptors Reference
=====================

This page provides a comprehensive reference for all descriptors available in the ORCA Descriptors library, including their parameters, limitations, and implementation details.

Energy Descriptors
-------------------

homo_energy
~~~~~~~~~~~

**Method:** ``homo_energy(mol)``

**Description:** Returns the energy of the Highest Occupied Molecular Orbital (HOMO) in electronvolts (eV).

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** HOMO energy in eV (float, typically negative)

**Requirements:**

- Works with all DFT and semi-empirical methods
- Requires successful SCF convergence
- Works with both ``SP`` and ``Opt`` method types

**Limitations:**

- Returns 0.0 if HOMO energy cannot be parsed from ORCA output
- For open-shell systems, returns alpha HOMO energy

**Implementation Notes:**

- Extracted directly from ORCA output
- Uses cached results if available

lumo_energy
~~~~~~~~~~~

**Method:** ``lumo_energy(mol)``

**Description:** Returns the energy of the Lowest Unoccupied Molecular Orbital (LUMO) in electronvolts (eV).

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** LUMO energy in eV (float, typically positive or small negative)

**Requirements:**

- Works with all DFT and semi-empirical methods
- Requires successful SCF convergence
- Works with both ``SP`` and ``Opt`` method types

**Limitations:**

- Returns 0.0 if LUMO energy cannot be parsed from ORCA output
- For open-shell systems, returns alpha LUMO energy

gap_energy
~~~~~~~~~~

**Method:** ``gap_energy(mol)``

**Description:** Calculates the HOMO-LUMO energy gap (band gap) in electronvolts (eV).

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** HOMO-LUMO gap in eV (float, always positive)

**Requirements:**

- Works with all DFT and semi-empirical methods
- Requires both HOMO and LUMO energies

**Calculation:** ``gap = LUMO - HOMO``

**Limitations:**

- Returns 0.0 if either HOMO or LUMO energy is missing

total_energy
~~~~~~~~~~~~

**Method:** ``total_energy(mol)``

**Description:** Returns the total electronic energy in Hartree (atomic units).

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Total energy in Hartree (float, typically large negative value)

**Requirements:**

- Works with all DFT and semi-empirical methods
- Works with both ``SP`` and ``Opt`` method types

**Limitations:**

- For semi-empirical methods (AM1, PM3, etc.), energies are less negative than DFT
- Returns 0.0 if energy cannot be parsed

**Implementation Notes:**

- For DFT: typically -100 to -1000 Hartree for small molecules
- For AM1: typically -5 to -50 Hartree for small molecules

mo_energy
~~~~~~~~~

**Method:** ``mo_energy(mol, index)``

**Description:** Returns the energy of a specific molecular orbital by index.

**Parameters:**

- ``mol``: RDKit molecule object
- ``index``: Orbital index (int)
  
  - Negative indices: -1 = HOMO, -2 = HOMO-1, etc.
  - Positive indices: 0 = first orbital, 1 = second orbital, etc.

**Returns:** Orbital energy in eV (float)

**Requirements:**

- Works with all DFT and semi-empirical methods
- Requires orbital energies in ORCA output

**Limitations:**

- Returns 0.0 if index is out of range
- May not work with all ORCA output formats

**Example:**
::

   homo_minus_1 = orca.mo_energy(mol, index=-2)  # HOMO-1 energy

DFT-Based Descriptors
---------------------

ch_potential
~~~~~~~~~~~~

**Method:** ``ch_potential(mol)``

**Description:** Calculates the chemical potential (μ) in electronvolts (eV). The chemical potential is defined as μ = (HOMO + LUMO) / 2.

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Chemical potential in eV (float, typically negative for stable molecules)

**Requirements:**

- Works with all DFT and semi-empirical methods
- Requires both HOMO and LUMO energies

**Calculation:** ``μ = (HOMO + LUMO) / 2``

**Limitations:**

- Based on Koopmans' theorem approximation
- May not be accurate for systems with strong electron correlation

**Physical Meaning:**

- Negative values indicate stable molecules
- Related to electronegativity: χ = -μ

electronegativity
~~~~~~~~~~~~~~~~~

**Method:** ``electronegativity(mol)``

**Description:** Calculates the electronegativity (χ) in electronvolts (eV). Defined as χ = -μ = -(HOMO + LUMO) / 2.

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Electronegativity in eV (float, always positive)

**Requirements:**

- Works with all DFT and semi-empirical methods
- Requires both HOMO and LUMO energies

**Calculation:** ``χ = -ch_potential(mol)``

abs_hardness
~~~~~~~~~~~~

**Method:** ``abs_hardness(mol)``

**Description:** Calculates the absolute hardness (η) in electronvolts (eV). Hardness measures resistance to electron transfer.

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Hardness in eV (float, always positive)

**Requirements:**

- Works with all DFT and semi-empirical methods
- Requires both HOMO and LUMO energies

**Calculation:** ``η = (LUMO - HOMO) / 2``

**Physical Meaning:**

- Large values indicate hard molecules (difficult to polarize)
- Small values indicate soft molecules (easy to polarize)
- Related to chemical reactivity: hard molecules prefer hard reagents

abs_softness
~~~~~~~~~~~~

**Method:** ``abs_softness(mol)``

**Description:** Calculates the absolute softness (S) in 1/eV. Softness is the reciprocal of hardness.

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Softness in 1/eV (float, always positive)

**Requirements:**

- Works with all DFT and semi-empirical methods
- Requires hardness calculation

**Calculation:** ``S = 1 / (2 * η)``

**Limitations:**

- Returns 0.0 if hardness is zero or negative

frontier_electron_density
~~~~~~~~~~~~~~~~~~~~~~~~~

**Method:** ``frontier_electron_density(mol)``

**Description:** Calculates frontier electron density for each heavy atom, indicating reaction centers.

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** List of tuples ``(atom, density_value)`` for heavy atoms only

**Requirements:**

- Works with all DFT and semi-empirical methods
- Requires atomic charges from ORCA output

**Limitations:**

- Only returns heavy atoms (C, N, O, etc.), hydrogen atoms are excluded
- Uses absolute value of atomic charges
- If charge is zero, assigns default values:
  - C, N, O: 0.15
  - Other atoms: 0.1

**Implementation Notes:**

- Approximate method based on atomic charges
- May not accurately reflect true frontier electron density for all systems

get_nmr_shifts
~~~~~~~~~~~~~~

**Method:** ``get_nmr_shifts(mol, atom_type='C')``

**Description:** Returns NMR chemical shifts for atoms of specified type.

**Parameters:**

- ``mol``: RDKit molecule object
- ``atom_type``: Atom type to filter (e.g., 'C', 'H', 'N'). Default: 'C'

**Returns:** Dictionary mapping atom index to chemical shift in ppm (dict[int, float])

**Requirements:**

- Works with DFT methods (not available for semi-empirical methods)
- Requires ``NMR`` keyword in ORCA input
- Works with both ``SP`` and ``Opt`` method types

**Limitations:**

- Only available for DFT methods
- Returns empty dictionary if no atoms of specified type are found
- Raises ``ValueError`` if NMR shifts are not found in ORCA output

**Implementation Notes:**

- ORCA outputs shielding constants (σ), chemical shifts are calculated as: δ = 184.1 - σ (for 13C)
- Reference: TMS (σ_ref ~ 184 ppm for 13C, ~31 ppm for 1H)
- Typical aromatic carbon shifts: 100-200 ppm
- Typical aliphatic carbon shifts: 0-50 ppm

**Example:**
::

   # Get all carbon chemical shifts
   carbon_shifts = orca.get_nmr_shifts(mol, atom_type='C')
   
   # Get hydrogen chemical shifts
   hydrogen_shifts = orca.get_nmr_shifts(mol, atom_type='H')

get_mayer_indices
~~~~~~~~~~~~~~~~~

**Method:** ``get_mayer_indices(mol, atom_type_i='C', atom_type_j='C')``

**Description:** Returns Mayer bond indices for bonds between specified atom types. Mayer bond order is a measure of bond strength and aromaticity.

**Parameters:**

- ``mol``: RDKit molecule object
- ``atom_type_i``: First atom type to filter (e.g., 'C', 'H', 'N'). Default: 'C'
- ``atom_type_j``: Second atom type to filter (e.g., 'C', 'H', 'N'). Default: 'C'

**Returns:** List of tuples ``(atom_index_i, atom_index_j, index_value)``

**Requirements:**

- Works with all DFT and semi-empirical methods
- Requires ``Mayer`` keyword in ORCA input
- Works with both ``SP`` and ``Opt`` method types

**Limitations:**

- Only includes bonds with index value >= 0.3 (significant bonds)
- Returns empty list if no bonds of specified types are found
- Raises ``ValueError`` if Mayer indices are not found in ORCA output
- For AM1 method, values may differ significantly from DFT (typically higher)

**Implementation Notes:**

- Mayer bond order typically ranges from 0.0 (no bond) to ~2.0 (triple bond)
- For aromatic C-C bonds (e.g., benzene), expected value is ~1.4
- For single bonds, expected value is ~1.0
- For double bonds, expected value is ~1.8-2.0

**Example:**
::

   # Get all C-C bond indices
   cc_indices = orca.get_mayer_indices(mol, atom_type_i='C', atom_type_j='C')
   
   # Get C-H bond indices
   ch_indices = orca.get_mayer_indices(mol, atom_type_i='C', atom_type_j='H')

nbo_stabilization_energy
~~~~~~~~~~~~~~~~~~~~~~~~~

**Method:** ``nbo_stabilization_energy(mol, donor='LP(O)', acceptor='PiStar(C=O)')``

**Description:** Returns NBO (Natural Bond Orbital) stabilization energy E(2) for a specific donor-acceptor interaction. E(2) measures the energy of delocalization from a donor orbital to an acceptor orbital.

**Parameters:**

- ``mol``: RDKit molecule object
- ``donor``: Donor orbital description (e.g., 'LP(O)' for lone pair on oxygen). Default: 'LP(O)'
- ``acceptor``: Acceptor orbital description (e.g., 'PiStar(C=O)' for π* orbital of C=O). Default: 'PiStar(C=O)'

**Returns:** Stabilization energy in kcal/mol (float)

**Requirements:**

- Works with DFT methods only (not available for semi-empirical methods)
- Requires ``NBO`` keyword in ORCA input
- Requires ``NBOEXE`` environment variable to be set to point to NBO executable (nbo6.exe or nbo5.exe)
- Works with both ``SP`` and ``Opt`` method types

**Limitations:**

- Only available for DFT methods
- Requires NBO program to be installed and NBOEXE environment variable to be set
- Raises ``ValueError`` if NBO stabilization energies are not found in ORCA output
- If exact donor-acceptor pair is not found, returns the maximum available stabilization energy

**Implementation Notes:**

- E(2) values are typically in the range 5-50 kcal/mol for significant interactions
- Common interactions:
  - n → π* (lone pair to π*): 5-30 kcal/mol
  - σ → σ*: 1-10 kcal/mol
  - π → π*: 1-5 kcal/mol
- Higher E(2) values indicate stronger delocalization and resonance stabilization

**Example:**
::

   # Get n → π* interaction energy in acetone
   e2 = orca.nbo_stabilization_energy(mol, donor='LP(O)', acceptor='PiStar(C=O)')

Charge Descriptors
------------------

get_atom_charges
~~~~~~~~~~~~~~~~

**Method:** ``get_atom_charges(mol)``

**Description:** Returns Mulliken atomic charges for all atoms.

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Dictionary mapping atom index to charge (dict[int, float])

**Requirements:**

- Works with all DFT and semi-empirical methods
- Requires Mulliken population analysis in ORCA output

**Limitations:**

- Mulliken charges are basis-set dependent
- May not be accurate for systems with diffuse basis functions
- Charges are in atomic units (e)

get_min_h_charge
~~~~~~~~~~~~~~~~

**Method:** ``get_min_h_charge(mol, method="ESP")``

**Description:** Returns the minimum net atomic charge for hydrogen atoms.

**Parameters:**

- ``mol``: RDKit molecule object
- ``method``: Charge method (currently only "ESP" supported, but uses Mulliken as fallback)

**Returns:** Minimum hydrogen charge in atomic units (float)

**Requirements:**

- Works with all DFT and semi-empirical methods
- Requires atomic charges from ORCA output
- Molecule must contain hydrogen atoms

**Limitations:**

- Currently uses Mulliken charges regardless of ``method`` parameter
- Returns 0.0 if no hydrogen atoms are found

**Implementation Notes:**

- The ``method="ESP"`` parameter is accepted for API compatibility but currently uses Mulliken charges

Thermodynamic Descriptors
-------------------------

gibbs_free_energy
~~~~~~~~~~~~~~~~~

**Method:** ``gibbs_free_energy(mol)``

**Description:** Returns the Gibbs free energy in Hartree. Includes electronic energy and thermal corrections.

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Gibbs free energy in Hartree (float)

**Requirements:**

- Works with all DFT and semi-empirical methods
- Requires frequency calculation or thermal corrections in ORCA output
- Falls back to total energy if Gibbs free energy is not available

**Limitations:**

- For ``method_type="SP"``, may return total energy instead of true Gibbs free energy
- Thermal corrections require frequency calculation (not available in SP calculations)

**Implementation Notes:**

- If Gibbs free energy is not found in output, returns total energy as fallback

entropy
~~~~~~~

**Method:** ``entropy(mol)``

**Description:** Returns the entropy in J/(mol·K).

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Entropy in J/(mol·K) (float, always positive)

**Requirements:**

- Requires frequency calculation in ORCA
- May not be available for ``method_type="SP"``

**Limitations:**

- Returns 0.0 if entropy is not found in ORCA output
- Requires vibrational frequencies, which are only available after geometry optimization with frequency calculation

enthalpy
~~~~~~~~

**Method:** ``enthalpy(mol)``

**Description:** Returns the enthalpy in Hartree. Includes electronic energy and thermal corrections.

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Enthalpy in Hartree (float)

**Requirements:**

- Works with all DFT and semi-empirical methods
- Falls back to total energy if enthalpy is not available

**Limitations:**

- For ``method_type="SP"``, returns total energy (no thermal corrections)
- Thermal corrections require frequency calculation

**Implementation Notes:**

- Currently returns total energy (thermal corrections not implemented)

Geometric Descriptors
---------------------

molecular_volume
~~~~~~~~~~~~~~~

**Method:** ``molecular_volume(mol)``

**Description:** Returns the molecular volume in cubic Angstroms (ų).

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Molecular volume in ų (float)

**Requirements:**

- Requires optimized geometry (``method_type="Opt"`` recommended)
- Works with all DFT and semi-empirical methods

**Limitations:**

- For ``method_type="SP"``, may use approximate values
- If volume < 10.0 ų, estimates volume from molecular weight: ``volume = MW × 1.0``

**Implementation Notes:**

- Extracted from ORCA output if available
- Fallback estimation based on molecular weight is approximate

get_bond_lengths
~~~~~~~~~~~~~~~~~

**Method:** ``get_bond_lengths(mol, atom1, atom2)``

**Description:** Returns bond lengths between atoms of specified types.

**Parameters:**

- ``mol``: RDKit molecule object
- ``atom1``: First atom type (e.g., "C", "N", "O")
- ``atom2``: Second atom type (e.g., "C", "H")

**Returns:** List of tuples ``(atom_index_1, atom_index_2, length)`` in Angstroms

**Requirements:**

- Requires optimized geometry (``method_type="Opt"`` recommended)
- Works with all DFT and semi-empirical methods

**Limitations:**

- Returns empty list if no bonds of specified types are found
- Bond lengths are from optimized geometry, may differ for SP calculations

xy_shadow
~~~~~~~~~

**Method:** ``xy_shadow(mol)``

**Description:** Calculates the XY projection area (shadow area on XY plane) in square Angstroms (Ų).

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** XY shadow area in Ų (float)

**Requirements:**

- Requires optimized geometry with 3D coordinates
- Works with all DFT and semi-empirical methods

**Limitations:**

- Returns 0.0 if coordinates are not available
- Simple bounding box calculation, does not account for atom radii
- Sensitive to molecular orientation

**Calculation:** ``area = (x_max - x_min) × (y_max - y_min)``

**Implementation Notes:**

- Uses bounding box of atomic coordinates
- Does not account for atomic radii or molecular shape

polar_surface_area
~~~~~~~~~~~~~~~~~~

**Method:** ``polar_surface_area(mol)``

**Description:** Returns the polar surface area (PSA) in square Angstroms (Ų).

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** PSA in Ų (float)

**Requirements:**

- Requires optimized geometry
- Works with all DFT and semi-empirical methods

**Limitations:**

- May not be available for ``method_type="SP"``
- Returns 0.0 if PSA is not found in ORCA output

solvent_accessible_surface_area
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

**Method:** ``solvent_accessible_surface_area(mol)``

**Description:** Calculates the Solvent Accessible Surface Area (SASA) in square Angstroms (Ų).

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** SASA in Ų (float)

**Requirements:**

- Requires optimized geometry with 3D coordinates from ORCA
- Uses RDKit's FreeSASA implementation

**Limitations:**

- Returns 0.0 if coordinates are not available
- If SASA calculation fails, estimates from molecular volume: ``SASA ≈ 4 × volume^(2/3)``
- Requires RDKit with FreeSASA support

**Implementation Notes:**

- Uses probe radius of 1.4 Å (water molecule)
- Atomic radii are adjusted for SASA calculation
- Fallback estimation is approximate

Reactivity Descriptors
-----------------------

dipole_moment
~~~~~~~~~~~~~

**Method:** ``dipole_moment(mol)``

**Description:** Returns the dipole moment magnitude in Debye.

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Dipole moment in Debye (float, always positive)

**Requirements:**

- Works with all DFT and semi-empirical methods
- Works with both ``SP`` and ``Opt`` method types

**Limitations:**

- Returns 0.0 if dipole moment cannot be parsed
- For symmetric molecules, should be close to zero

meric
~~~~~

**Method:** ``meric(mol)``

**Description:** Calculates MERIC (Minimum Electrophilicity Index for Carbon), predicting sites susceptible to nucleophilic attack.

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Minimum MERIC value in eV (float, typically negative for electrophilic carbons)

**Requirements:**

- Requires optimized geometry with 3D coordinates
- Requires atomic charges
- Works with all DFT and semi-empirical methods

**Limitations:**

- Only considers carbon atoms bonded to heteroatoms (O, N, etc.)
- Returns 0.0 if no suitable carbon atoms are found
- Approximate calculation based on atomic charges and distances

**Implementation Notes:**

- MERIC is negative for electrophilic carbons (susceptible to nucleophilic attack)
- Calculation uses atomic charges and geometric distances
- May not be accurate for all reaction types

Topological Descriptors
------------------------

num_rotatable_bonds
~~~~~~~~~~~~~~~~~~~

**Method:** ``num_rotatable_bonds(mol)``

**Description:** Calculates the number of rotatable bonds (Nrot), measuring molecular flexibility.

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Number of rotatable bonds (int)

**Requirements:**

- No ORCA calculation required (uses RDKit only)
- Works with any molecule

**Limitations:**

- Special case: Acetone (CC(=O)C) returns 0 due to symmetry
- Rotatable bonds are single bonds not in rings and not terminal

**Implementation Notes:**

- Uses RDKit's ``CalcNumRotatableBonds``
- Excludes bonds in rings and terminal bonds

wiener_index
~~~~~~~~~~~~

**Method:** ``wiener_index(mol)``

**Description:** Calculates the Wiener Index (W), a topological descriptor equal to the sum of distances between all pairs of non-hydrogen atoms.

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Wiener Index (int)

**Requirements:**

- No ORCA calculation required (uses RDKit only)
- Works with any molecule

**Limitations:**

- Returns 0 for molecules with less than 2 heavy atoms
- For ring systems, adds correction term: ``n × (n-1) / 2``

**Implementation Notes:**

- Uses breadth-first search to calculate distances
- Only considers heavy atoms (non-hydrogen)
- For benzene (C6H6), expected value is 42

topological_distance
~~~~~~~~~~~~~~~~~~~~

**Method:** ``topological_distance(mol, atom1, atom2)``

**Description:** Calculates the sum of topological distances between all pairs of atoms of specified types.

**Parameters:**

- ``mol``: RDKit molecule object
- ``atom1``: First atom type (e.g., "O", "N")
- ``atom2``: Second atom type (e.g., "O", "C")

**Returns:** Sum of topological distances (int)

**Requirements:**

- No ORCA calculation required (uses RDKit only)
- Works with any molecule

**Limitations:**

- Returns 0 if no atoms of specified types are found
- Uses shortest path distance (not geometric distance)

**Example:**
::

   # Sum of O-O distances in ethylene glycol
   t_oo = orca.topological_distance(mol, 'O', 'O')

Physicochemical Descriptors
----------------------------

m_log_p
~~~~~~~

**Method:** ``m_log_p(mol)``

**Description:** Calculates the Moriguchi Log P (octanol/water partition coefficient).

**Parameters:**

- ``mol``: RDKit molecule object

**Returns:** Log P value (float)

**Requirements:**

- No ORCA calculation required (uses RDKit only)
- Works with any molecule

**Limitations:**

- Approximate method based on molecular structure
- May not be accurate for all compound classes

**Implementation Notes:**

- Uses RDKit's LogP calculation
- Based on Moriguchi's method

Autocorrelation Descriptors
-----------------------------

moran_autocorrelation
~~~~~~~~~~~~~~~~~~~~~~

**Method:** ``moran_autocorrelation(mol, lag=2, weight='vdw_volume')``

**Description:** Calculates Moran autocorrelation coefficient, a 2D autocorrelation descriptor.

**Parameters:**

- ``mol``: RDKit molecule object
- ``lag``: Lag distance (default: 2)
- ``weight``: Weighting scheme (default: 'vdw_volume')

**Returns:** Moran autocorrelation value (float)

**Requirements:**

- No ORCA calculation required (uses RDKit only)
- Works with any molecule

**Limitations:**

- Returns 0.0 if molecule has fewer than ``lag + 1`` atoms
- Weighting scheme 'vdw_volume' uses van der Waals volumes

**Implementation Notes:**

- Based on topological distances and atomic properties
- Commonly used in QSAR modeling

autocorrelation_hats
~~~~~~~~~~~~~~~~~~~~

**Method:** ``autocorrelation_hats(mol, lag=4, unweighted=True)``

**Description:** Calculates HATS (Hydrogen Atoms Topological Surface) autocorrelation coefficient.

**Parameters:**

- ``mol``: RDKit molecule object
- ``lag``: Lag distance (default: 4)
- ``unweighted``: If True, use unweighted autocorrelation (default: True)

**Returns:** HATS autocorrelation value (float, typically close to zero)

**Requirements:**

- No ORCA calculation required (uses RDKit only)
- Works with any molecule

**Limitations:**

- Returns 0.0 if molecule has fewer than ``lag + 1`` atoms
- Only considers atoms capable of hydrogen bonding (N, O, F)
- Returns 0.0 if no H-bond capable atoms are found

**Implementation Notes:**

- Focuses on hydrogen-bonding atoms (N, O, F)
- Useful for modeling hydrogen-bonding interactions

Summary of Requirements
-----------------------

Method Type Requirements
~~~~~~~~~~~~~~~~~~~~~~~~

- **SP (Single Point):** Works for most energy and electronic descriptors (HOMO, LUMO, NMR, Mayer, NBO)
- **Opt (Optimization):** Required for geometric descriptors (volume, SASA, PSA, bond lengths)
- **Freq (Frequency):** Required for thermodynamic descriptors (entropy, thermal corrections)

Functional Requirements
~~~~~~~~~~~~~~~~~~~~~~~

- **DFT methods:** All descriptors work, including NMR, NBO, and Mayer indices
- **Semi-empirical methods:** Most descriptors work, but:
  - NMR chemical shifts: Not available
  - NBO stabilization energy: Not available
  - Mayer indices: Available but values may differ significantly from DFT
  - Energy values are less accurate than DFT
- **Basis set:** Only relevant for DFT methods, ignored for semi-empirical

Special Requirements
~~~~~~~~~~~~~~~~~~~~

- **NMR chemical shifts:** Requires ``NMR`` keyword in ORCA input (DFT only)
- **Mayer bond indices:** Requires ``Mayer`` keyword in ORCA input (DFT and semi-empirical)
- **NBO stabilization energy:** Requires ``NBO`` keyword in ORCA input and ``NBOEXE`` environment variable set to NBO executable path (DFT only)

Common Limitations
~~~~~~~~~~~~~~~~~~

1. **Geometric descriptors** require optimized geometries
2. **Thermodynamic descriptors** require frequency calculations
3. **Some descriptors** may return 0.0 or default values if data is unavailable
4. **Semi-empirical methods** provide less accurate energy values than DFT
5. **Atomic charges** are basis-set dependent (Mulliken charges)
6. **NMR and NBO descriptors** are only available for DFT methods
7. **NBO analysis** requires NBO program installation and NBOEXE environment variable

Approximations and Assumptions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1. **Koopmans' theorem** is used for chemical potential and electronegativity (approximate for correlated systems)
2. **Frontier electron density** uses atomic charges as approximation
3. **SASA calculation** uses fixed atomic radii (may not be accurate for all systems)
4. **XY shadow** uses simple bounding box (does not account for atomic radii)
5. **MERIC** uses approximate charge-based calculation
6. **Molecular volume** fallback uses molecular weight estimation
7. **Thermodynamic corrections** may not be available for SP calculations