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. 2021 Nov 21;155(19):194103.
doi: 10.1063/5.0072380.

Improvement of the Gaussian Electrostatic Model by separate fitting of Coulomb and exchange-repulsion densities and implementation of a new dispersion term

Sehr Naseem-Khan  1 , Jean-Philip Piquemal  2 , G Andrés Cisneros  1
Affiliations

Improvement of the Gaussian Electrostatic Model by separate fitting of Coulomb and exchange-repulsion densities and implementation of a new dispersion term

Sehr Naseem-Khan et al. J Chem Phys. .

Abstract

The description of each separable contribution of the intermolecular interaction is a useful approach to develop polarizable force fields (polFFs). The Gaussian Electrostatic Model (GEM) is based on this approach, coupled with the use of density fitting techniques. In this work, we present the implementation and testing of two improvements of GEM: the Coulomb and exchange-repulsion energies are now computed with separate frozen molecular densities and a new dispersion formulation inspired by the Sum of Interactions Between Fragments Ab initio Computed polFF, which has been implemented to describe the dispersion and charge-transfer interactions. Thanks to the combination of GEM characteristics and these new features, we demonstrate a better agreement of the computed structural and condensed properties for water with experimental results, as well as binding energies in the gas phase with the ab initio reference compared with the previous GEM* potential. This work provides further improvements to GEM and the items that remain to be improved and the importance of the accurate reproduction for each separate contribution.

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Figures

FIG. 1.
FIG. 1.
Schematic representation of the development of GEM. (a) GEM-0, (b) GEM*, and (c) GEM.
FIG. 2.
FIG. 2.
Comparison of the Coulomb, exchange-repulsion, disp+ct, and binding energies computed with GEM* and GEM with SAPT(DFT) for the ten Smith dimers. For computational details of the three disp+ct reference curves, see Eqs. (8)–(10) in the main text. (a) Coulomb, (b) exchange-repulsion, (c) disp+CT, and (d) binding energy.
FIG. 3.
FIG. 3.
Energy components computed with GEM* and GEM for the linear scan of the water dimer. For computational details of the three disp+ct reference curves, see Eqs. (8)–(10) in the main text. (a) Coulomb, (b) exchange-repulsion, (c) polarization, (d) disp+CT, and (e) binding energy.
FIG. 4.
FIG. 4.
Energy components computed with GEM* and GEM for the angular scan of the canonical water dimer (Y-axis) at the fixed equilibrium distance O⋯H of 1.9 Å. For computational details of the three disp+ct reference curves, see Eqs. (8)–(10) in the main text. (a) Coulomb, (b) exchange-repulsion, (c) polarization, (d) disp+CT, and (e) binding energy.
FIG. 5.
FIG. 5.
Comparison of binding energies computed with GEM* and GEM with the CCSD(T)/CBS for the water clusters: (a) trimers, (b) tetramers, (c) pentamers, and (d) hexamers.
FIG. 6.
FIG. 6.
Comparison of the Radial Distribution Functions (RDFs) with the experimental data at 300 K (left panel) and the Spatial Distribution Functions (SDFs) in three directions computed with GEM. The pink isosurfaces represent the oxygen atom and its lone pairs, while the white isosurfaces represent the hydrogen atoms (right panel). (a) RDF (O–O), (b) SDF (xy), (c) RDF (O–H), (d) SDF (xz), (e) RDF (H–H), and (f) SDF (yz).
FIG. 7.
FIG. 7.
Condensed phase properties computed with GEM* and GEM compared to the experiment. (a) Density, (b) enthalpy of vaporization, and (c) self-diffusion coefficient.
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