A simple potential model criterion for the quality of atomic charges

The simple potential model has been shown to be useful in relating core ele ctron binding energies measured in the X-ray region with mean dipole moment derivatives obtained from experimental infrared vibrational intensities. T he importance of including relaxation corrections to the experimental Is io nization energies of sp, sp(2), and sp(3) hybridized carbon atoms are inves tigated here. Although relaxation energies obtained from 6-31G(d,p) and 6-3 11++G(3df,3p) basis sets using Delta SCF calculations show differences of a bout 1 eV for most molecules studied, relative differences are of the order of 0.1 eV. Exceptions are the CO, CO2, COS, and CS2 molecules where discre pancies are larger. Relaxation energy corrections improve simple potential model fits with mean dipole moment derivatives for all carbon atom models b ut is most pronounced for the sp hybridized atoms. The simple potential mod el corrected for relaxation energies is investigated as a criterion for tes ting the quality of Mulliken, CHELPG, Bader and GAPT carbon atomic charges calculated from MP2/6-311++G(3d,3p) wave functions. The GAPT charges are in excellent agreement with the experimental mean dipole moment derivatives ( within 0.067e) and provide superior statistical fits to the simple potentia l model when compared with those obtained for the ether charges.