Transferable Group Contributions for a Variety of Chemical Phenomena and Compound

An approximate method for calculating molecular electrostatic potential (MEP) maps and atomic point charge models for large molecules in a reduced computational time is proposed and tested for two widely used basis sets (STO-3G and 6-31G*). The method avoids the molecular orbital calculation of the whole system by expressing its first order electronic density matrix in terms of transferable localized orbitals (TLO), previously determined on model molecules, via a localization process followed by the cutting of the tails, and stored in two databases (one for each basis set). For systems with a canonic electronic structure TLO are made of a single vector, involving either two nuclei (to describe the covalent bond between those atoms) or one nucleus (to describe lone pairs and inner shells). Conversely, delocalized p systems require many-center TLO, formed by a suitable number of vectors. Density functions of large chemical compounds can thus be built up automatically from a code that recognizes which fragments are contained in the system of interest, extracts them from the chosen database, reorders the atoms consistently with the pertinent TLO and places them in the correct position and orientation on the relevant atoms. A great number of chemical groups were parameterized and the efficiency of the method was evaluated on different systems, including aliphatic hydrocarbons. Numerical calculations on several molecules revealed that this approximation brought no significant loss of accuracy with respect to the corresponding HF values for the examined properties. Though the method is specifically designed to produce approximate wavefunctions, the point charge models obtained by fitting the corresponding MEP represent a viable alternative when ab initio HF calculations are not affordable and can be used in connection to any popular force field.