11.5.7 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 11.10. The fragment library is located in $QCAUX/fraglib directory. gamess format of EFPs is used.

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 with 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.  378 Flick J. C. et al.
J. Chem. Theory Comput.
(2012), 8, pp. 2835. Link . 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.