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High-Precision Quantum Thermochemistry on Nonquasiharmonic Potentials: Converged Path-Integral Free Energies and a Systematically Convergent Family of Generalized Pitzer−Gwinn Approximations

2005; American Chemical Society; Volume: 109; Issue: 44 Linguagem: Inglês

10.1021/jp051742n

1520-5215

Vanessa Audette Lynch, Steven L. Mielke, Donald G. Truhlar,

Spectroscopy and Quantum Chemical Studies

Accurate quantum mechanical (QM) vibrational−rotational partition functions for HOOD, D2O2, H18OOH, H218O2, D18OOH, and H18OOD are determined using a realistic potential energy surface for temperatures ranging from 300 to 2400 K by using the TT-FPI-ESPE path-integral Monte Carlo method. These data, together with our prior results for H2O2, provide benchmarks for testing approximate methods of estimating isotope effects for systems with torsional motions. Harmonic approximations yield poor accuracy for these systems, and although the well-known Pitzer−Gwinn (PG) approximation provides better results for absolute partition functions, it yields the same results as the harmonic approximation for isotope effects because these are intrinsically quantal phenomena. We present QM generalizations of the PG approximation that can provide high accuracy for both isotope effects and absolute partition functions. These approximations can be systematically improved until they approach the accurate result and converge rapidly. These methods can also be used to obtain affordable estimates of zero-point energies from accurate partition functionseven those at relatively high temperatures.