Polarized Molecular Orbital Model Chemistry 3. The PMO Method Extended to Organic Chemistry

Abstract

The polarized molecular orbital (PMO) method, a neglect-of-diatomic-differential-overlap (NDDO) semiempirical molecular orbital method previously parameterized for systems composed of O and H, is here extended to carbon. We modified the formalism and optimized all the parameters in the PMO Hamiltonian by using a genetic algorithm and a database containing both electrostatic and energetic properties; the new parameter set is called PMO2. The quality of the resulting predictions is compared to results obtained by previous NDDO semiempirical molecular orbital methods, both including and excluding dispersion terms. We also compare the PMO2 properties to SCC-DFTB calculations. Within the class of semiempirical molecular orbital methods, the PMO2 method is found to be especially accurate for polarizabilities, atomization energies, proton transfer energies, noncovalent complexation energies, and chemical reaction barrier heights and to have good across-the-board accuracy for a range of other properties, including dipole moments, partial atomic charges, and molecular geometries.

Quantum mechanical force field for hydrogen fluoride with explicit electronic polarization

“…58,59 Three key modifications were introduced in PMO. Since the method has been reported in detail previously, 35,[51][52][53] we only provide a brief summary of its key departures from MNDO.…”

“…At present, three different variations of the PMO method have been reported, PMOv1, 52 PMO2, 53 and PMOw, 35 none of which contain parameters for fluorine. The variant of PMO for which we have decided to introduce the F parameters is the PMOw model, having kept all other parameters fixed to those reported in Ref.…”

“…As in earlier quantum studies of HF, 29,43 a two-site monomer is used in the X-Pol model for HF (XPHF), as opposed to the more common threesite approach in classical models. In the present study, we employ the "semiempirical" polarized molecular orbital (PMO) method, [51][52][53] by introducing a set of p-orbitals onto the hydrogen atom. The performance of the PMO model on molecular polarization and hydrogen-bonding is significantly improved over existing semiempirical models.…”

Quantum mechanical force field for water with explicit electronic polarization

“…[44][45][46][47][48][49] Another important deficiency of many semiempirical models for treating nonbonded interactions is that molecular polarization is systematically underestimated. Recently, we have introduced a polarized molecular orbital (PMO) alternative, [49][50][51] in which a set of p-orbitals are added to each hydrogen atom. 52 It was found that the computed molecular polarizabilities for a range of compounds containing hydrogen, carbon, and oxygen are significantly improved.…”

“…52 It was found that the computed molecular polarizabilities for a range of compounds containing hydrogen, carbon, and oxygen are significantly improved. 49,51 Employing this strategy, we report here a parametrization of the PMO model for water (PMOw), which can be used in X-Pol for liquid simulations.…”