Automated Rovibrational Configuration Interaction Calculations

The calculation of rovibrational infrared spectra of molecules without the use of model Hamiltonians is a demanding task, which includes several computational bottlenecks and requires the monitoring of intermediate quantities. This chapter outlines the computational workflow of an implementation of rovibrational configuration interaction (RVCI) theory in the MOLPRO package of ab initio programs, which requires just a bare minimum of information from the user to generate a high-quality spectrum, which pops out at the end. The RVCI wavefunction can be expressed in terms of products of pure vibrational and rotational basis functions. Vibrational configuration interaction (VCI) wavefunctions as obtained from a configuration-selective implementation provide a compact and optimized vibrational basis. The dimension of the rovibrational basis grows rapidly depending on the number of vibrational basis functions and the angular momentum quantum number. The VCI calculations have been automated to large extent and little expert knowledge is needed nowadays to yield reliable results.