Analytical potentials from Ab initio computations for hydrations of methylamine and methylammonium ion

Abstract

The interaction energy between water ($H_2O$) and methylamine ($CH_3NH_2$) and methylammonium ion ($CH_3NH_3+$), respectively, is of interest because these are side chains of proteins and nucleic acid. A crucial aspect in these studies is the availability of intermolecular potential functions for Monte Carlo simulations and structure, energetics of these molecules in water solvent. Hence, we can consider the protein hydrations theoretically. The water-solute interaction energies were considered by ab initio SCF calculation. We used 6-31G basis set, and the monomer geometries of $CH_3NH_2$, $CH_3NH_3+$ and $H_2O$ are kept rigid, therefore the bond length and the bond angles are taken from experimental values. The computed ab initio energies of the dimers had been used to derive a simple analytic pair potential function of the form $\Sigma_i\,\Sigma_j(-A_{ab}\mid{r}_{ij}^6+B_{ab}\mid{r}_{ij}^{12}+Q_iQ_i\mid{r}_{ij}$ type that reproduce the Hartree-Fock potential energy surfaces. We optimized the solute-water dimer structure by our fitted analytic potential functions to test the availability of our functions and this optimized geometric structures are compared with ab initio optimized structures that are computed with the 6-31G basis set.