Psi4 is an open-source quantum chemistry package integrated into ICM for calculating molecular energies, optimizing molecular geometries, and assigning atomic partial charges. It is bundled with ICM and runs internally, so no separate installation or configuration is required.
The integration provides three main capabilities:
- Geometry optimization: Optimize molecular structures using density functional theory (DFT), including individual molecules, conformational stacks, and 3D chemicals in tables.
- Single-point energy calculation: Calculate the DFT energy of a molecular structure without changing its geometry.
- Atomic partial charge calculation: Calculate restrained electrostatic potential (RESP) charges by fitting atomic charges to the quantum electrostatic potential.
The default method for geometry optimization and energy calculation is B3LYP/6-31G*. Alternative functionals and basis sets can be specified via the command line. RESP charge calculations use HF/6-31G* by default.
ICM converts quantum energies from Hartree units to kcal/mol. For conformational stacks, or table entries sharing the same unique SMILES, energies are reported relative to the lowest-energy conformer.
Calculation time increases with molecular size and the size of the basis set. The number of CPU cores used is controlled by the nProc setting in the command line or Number of Cores in the GUI.