Modeling of the Exchange Repulsion Energy

Abstract

The nonempirical force field model (NEMO) has been augmented with a new exchange repulsion model based on a different choice of expansion center. In the new repulsion model, the size of the atoms are estimated from the trace of the local second-order electron density tensor divided with the local valence charge. A set of general atom type dependent parameters are fitted from SCF calculations on different complexes. An exponential form is used to describe the interaction. The new expansion center is chosen as the point where the local electronic dipole is zero, i.e. the center of electronic charge. The model has been tested on a set of intermolecular complexes between the molecules water, ammonia, methylamine, formamide, and urea.

Inter‐ and intramolecular potential for the N‐formylglycinamide‐water system. A comparison between theoretical modeling and empirical force fields

“…The hydrogen bond distances for the structures in the figures are all, in general, shorter according to the NEMO method than what is normally seen for hydrogen bonding systems. We have observed, from previous work, that the NEMO model can give too compact structures, 6,15,32 but the introduction of dispersion is known to produce shorter distances than those obtained at RHF SCF level. 33 Accordingly, if one calculates the SCF part of the NEMO potential at the optimized NEMO geometry, one would expect these energies to be less binding than the corresponding energies obtained by the ab initio SCF method at the ab initio SCF optimized geometries.…”

“…6 This creates problems when the dihedral angles are changed in a molecule because these angles are defined using the atomic positions. The straightforward solution is to introduce the nuclear positions in the description of the molecule.…”

“…However, for the evaluation of the repulsion energy the second moments are explicitly used. 6 Furthermore, all atomic dipoles in FG are modified to correct for the internal fields originating from parts of the molecule that move relative to the considered atom. The atomic dipole moment obtained from a MME contains an induced part due to the field from the other atoms in the molecule.…”

“…This has driven the development of modeling molecular interactions by using analytical potential functions to represent the inter-and intramolecular forces. [1][2][3][4][5][6] However, most of the ab initio potentials are constructed for rigid molecules, because the angular dependency and complexity of the intramolecular energy imply that many parameters are needed to fit the dihedral potential obtained from ab initio calculations or experimental data. Already, with two dihedral angles in a molecule, the phase space needed to be covered becomes large, and for molecules with four or five dihedral angles the task becomes, in practice, impossible.…”

“…The potential used in this work is of NEMO type. 1,6 This type of potential was used for flexible molecules in a previous work. 15 The work presented in this article represents a first step in the construction of flexible inter-and intramolecular potentials suitable for the modeling of proteins based on quantum chemical calculations.…”