12.5.5 Library of Fragments

The effective fragments are rigid and their potentials are generated from a set of ab initio calculations on each unique isolated fragment. The EFP includes: ($i$) multipoles (produced by the Stone’s Distributed Multipolar Analysis) for Coulomb and polarization terms; ($ii$) static polarizability tensors centered at localized molecular orbital (LMO) centroids (obtained from coupled-perturbed Hartree-Fock calculations), which are used for calculations of polarization; ($iii$) dynamic polarizability tensors centered on the LMOs that are generated by time-dependent HF calculations and used for calculations of dispersion; and ($iv$) the Fock matrix, basis set, and localized orbitals needed for the exchange-repulsion term. Additionally, the EF potential contains coordinates of atoms, coordinates of the points of multi-polar expansion (typically, atoms and bond mid-points), coordinates of the LMO centroids, electrostatic and polarization screening parameters, and atomic labels of the EF atoms.

Q-Chem provides a library of standard fragments with precomputed effective fragment potentials. Currently the library includes common organic solvents, nucleobases, and molecules from S22 and S66 datasets for non-covalent interactions; see Table 12.8. EFP potentials in gamess format are supported by new EFPMAN2 module. They are stored in $QCAUX/fraglib directory.

Note:  The fragments from Q-Chem fragment library have _L added to their names to distinguish them from user-defined fragments.

The parameters for the standard fragments were computed as follows. The geometries of the solvent molecules were optimized by MP2/cc-pVTZ; geometries of nucleobases were optimized with RI-MP2/cc-pVTZ. Geometries of molecules from S22 and S66 datasets are discussed in Ref. 265. The EFP parameters were obtained in gamess. To generate the electrostatic multipoles and electrostatic screening parameters, analytic DMA procedure was used, with 6-31+G* basis for non-aromatic compounds and 6-31G* for aromatic compounds and nucleobases. The rest of the potential, i.e., static and dynamic polarizability tensors, wave function, Fock matrix, etc., were obtained using 6-311++G(3df,2p) basis set.