Anharmonic Force Fields and Thermodynamic Functions using Density Functional Theory

The very good performance of modern density functional theory for molecular geometries and harmonic vibrational frequencies has been well established. We investigate the performance of density functional theory (DFT) for quartic force fields, vibrational anharmonicity and rotationvibration coupling constants, and thermodynamic functions beyond the RRHO (rigid rotorharmonic oscillator) approximation of a number of small polyatomic molecules. Convergence in terms of basis set, integration grid and the numerical step size for determining the quartic force field by using central differences of analytical second derivatives has been investigated, as well as the performance of various exchange-correlation functionals. DFT is found to offer a costeffective approach with manageable scalability for obtaining anharmonic molecular properties, and particularly as a source for anharmonic zero-point and thermal corrections for use in conjunction with benchmark ab initio thermochemistry methods. ∗ Dedicated to Prof. Nicholas C. Handy FRS on the occasion of his 63rd birthday. Electronic address: [email protected]