MOLECULAR SPECTROSCOPY FROM FIRST PRINCIPLES

Over the past years there have been a rapid improvement in nuclear motion approaches to solving spectroscopic problems, which has been described as the fourth age of quantum chemistry.\footnote{A. G. Cs\'asz\'ar, C. Fabri, T. Szidarovszky, E. M\'atyus, T. Furtenbacher, and G. Czak\'o, \textit{Phys. Chem. Chem. Phys.} \textbf{14}, 1085 (2012).} The methodology which is commonly attributed to the spectroscopy from first principles is in fact a combination of high level \textit{ab initio} (electronic structure) calculations, high level nuclear motion (variational) calculations and empirical refinement to the highly accurate experimental data (e.g. line positions).\footnote{J. Tennyson and S.N. Yurchenko , \textit{Int. J. Quant. Chem. } \textbf{117}, 92, (2017).} In this talk, I will discuss the current state-of-the-art of the theoretical molecular spectroscopy, which shows that this methodology is increasingly competitive with experiment and allows in many cases a more reliable determination of various molecular data.\footnote{J. Tennyson, \textit{J. Chem. Phys.} \textbf{145}, 120901 (2016).} Our variational program \textsc{TROVE}\footnote{S.N. Yurchenko, W. Thiel, P. Jensen, \textit{J. Mol. Spectrosc.} \textbf{245}, 126–140 (2007).} is one of the modern computational tools successfully used to generate huge lists of transitions which provide the input for models of atmospheric absorption. I will review the methodology used by \textsc{TROVE} and other variational programs for accurate solution of the nuclear motion Schr\"{o}dinger equation for general medium size polyatomic molecules, show examples of successful applications and discuss cases, which are still challenging for the modern \textit{ab initio} methods