Computation of quartic potential energy surfaces and excited vibrational levels of H2O

COMP 217

Nicole R. Brinkmann, John L. Davisson, Michael H. Cortez, and William F. Polik. Department of Chemistry, Hope College, Holland, MI 49423

The quartic potential energy surface of H2O was computed using the coupled-cluster singles, doubles, and perturbatively-applied triples [CCSD(T)] level of theory.  The cc-pVXZ, aug-cc-pVXZ, and aug-cc-pCVXZ (X = D-5) families of basis sets were used for these computations.  Harmonic frequencies, anharmonic constants, and resonance constants were computed using second-order vibrational perturbation theory (VPT2) including the k11,33 Darling-Dennison resonance, and vibrational energy levels were predicted using these (harmonic, anharmonic, and resonance) spectroscopic constants.  The spectroscopic constants were fit to experimental vibrational energy levels, and the standard deviation of the fit was 3.7 cm-1.  The average absolute difference between the theoretical harmonic frequencies and the fitted harmonic frequencies (‹|error|›) was used to gauge the accuracy of the computations.  The (‹|error|›) quantity for computed harmonic frequencies and anharmonic constants at a particular basis set were compared to that with the next smallest basis set by an average absolute difference (‹|D|›) as a measure of convergence.  Using the aug-cc-pVXZ basis sets, the harmonic frequencies are converged to less than 3 cm-1 at X = 4 (‹|error|›QZ = 2.76 cm-1;‹|D5Z-QZ|› = 1.29 cm-1).  The anharmonic constants converge more rapidly, and using the cc-pVXZ series of basis sets, they are converged to less than 2 cm-1 at X = T (‹|error|›TZ = 1.89 cm-1;‹|DQZ-TZ|› = 0.03 cm-1).  Thus, a scheme was devised for a set of “hybrid” computations in which the harmonic part of the potential was computed using CCSD(T)/aug-cc-pVQZ while the more expensive cubic and quartic potential terms were computed using CCSD(T)/cc-pVTZ, and the vibrational energy levels were predicted using this hybrid potential energy surface.  Using this aug-cc-pVQZ/cc-pVTZ hybrid model, ‹|error|› = 3.56 cm-1 and 2.31 cm-1 for the harmonic frequencies and anharmonic constants, respectively.  Results for H2CO, HFCO, HCO are also presented.