PROJECTS

Development and application of explicitly-correlated coupled-cluster methods
for nonlinear optical properties and excited states

PRINCIPAL INVESTIGATOR

Prof. Dr. Christof Hättig Ruhr-Universität Bochum Lehrstuhl für Theoretische Chemie Universitätsstraße 150 44780 Bochum Germany 0234/3228082 0234/3214045 christof.haettig@theochem.ruhr-uni-bochum.de www: Homepage

TOGETHER WITH

Prof. Dr. Willem Maarten Klopper Universität Karlsruhe Institut für Physikalische Chemie Kaiserstraße 12 76131 Karlsruhe Germany 0721/6087263 0721/6083319 wim.klopper@chemie.uni-karlsruhe.de www: Homepage

PROJECT RESEARCH ASSISTANT

ABSTRACT

Coupled-cluster (CC) response theory has during the last decade become a standard approach for accurate calculations of optical properties. These are presently the only methods allowing to calculate NLO properties of many-electron systems with an accuracy of a few percent. As other correlated wavefunction approaches, CC response methods are limited in their applicability by the slow basis set convergence and the steep increase of the computational costs with basis set size. The goal of this project is to combine CC response theory with the R12 ansatz (and its F12 generalization) to overcome this problem and make these methods applicable to larger molecules and increasing their accuracy. During the previous application period we have implemented CC-R12 response theory at the level of the CCSD(R12) model. First results for excited states and (hyper-) polarizabilities demonstrate a significant acceleration of the basis set convergence for these properties with the R12 ansatz. The aim for the next application period will be to extend the developed CCSD(R12) response code to the more accurate F12 variant, which uses a modified correlation factor and a different projection operator for the pair functions (so-called ansatz 2), to fully exploit the improved basis set convergence of these newly devised explicitly-correlated methods. Another objective will be the development of approximations to ansatz 2 to reduce the computational costs of calculations for excited states with the CC2-F12 model.