Software news and updates - DOIT: A program to calculate thermal rate constants and mode-specific tunneling splittings directly from quantum-chemicalcalculations

In this contribution we discuss computational aspects of a recently introdu ced method for the calculation of proton tunneling rate constants, and tunn eling splittings, which has been applied to molecules and complexes, and sh ould apply equally well to bulk materials. The method is based on instanton theory, adapted so as to permit a direct link to the output of quantum-che mical codes. It is implemented in the DOIT (dynamics of instanton tunneling ) code, which calculates temperature-dependent tunneling rate constants and mode-specific tunneling splittings. As input, it uses the structure, energ y, and vibrational force field of the stationary configurations along the r eaction coordinate, computed by conventional quantum-chemical programs. The method avoids the difficult problem of calculating the exact least-action trajectory, known as the instanton path, and instead focusses on the corres ponding instanton action, because it governs the dynamic properties. To app roximate this action for a multidimensional system, the program starts from the one-dimensional instanton action along the reaction coordinate, which can be obtained without difficulty. It then applies correction terms for th e coupling to the other vibrational degrees of freedom, which are treated a s harmonic oscillators (transverse normal modes). The couplings are assumed linear in these modes. Depending on the frequency and the character of the transverse modes, they may either decrease or increase the action, i.e., h elp or hinder the transfer. A number of tests have shown that the program i s at least as accurate as alternative programs based on transition-state th eory with tunneling corrections, and is also much less demanding in compute r time, thus allowing application to much larger systems. An outline of the instanton formalism is presented, some new developments are introduced, an d special attention is paid to the connection with quantum-chemical codes. Possible sources of error are investigated. To show the program in action, calculations are presented of tunneling rates and splittings associated wit h triple proton transfer in the chiral water trimer. (C) 2001 John Wiley & Sons, Inc.