%0 Journal Article %1 publ9340800 %A Celani, Paolo %A Werner, Hans Joachim %D 2003 %J J. Chem. Phys. %K chemie imported werner from:alexanderdenzel theoretische stuttgart theochem %N 10 %P 5044–5057 %R 10.1063/1.1597672 %T Analytical energy gradients for internally contracted second-order multireference perturbation theory %U http://dx.doi.org/10.1063/1.1597672 %V 119 %X The theory for computing analytical energy gradients for second-order multireference perturbation theory MRPT2 with arbitrary multiconfiguration self-consistent field MCSCF reference functions is derived and implemented. MRPT2 with complete active space reference functions CASPT2 is a special case. In our method the configurations with two electrons in the external orbital space are internally contracted. This ansatz strongly reduces the length of the configuration expansion as compared to uncontracted wave functions, but avoids bottlenecks occurring when fully contracted first-order wave functions are used. The method, which also allows the use of state-averaged MCSCF reference functions, is applied to optimize the structures of low-lying valence and Rydberg states of Pyrrole. Nonplanar equilibrium structures and large geometry relaxation effects on the excitation energies are found for the valence states. From the results it can be concluded that CASPT2 underestimates the excitation energies of the valence states, but leads to accurate results for the Rydberg states.

@article{publ9340800, abstract = {The theory for computing analytical energy gradients for second-order multireference perturbation theory MRPT2 with arbitrary multiconfiguration self-consistent field MCSCF reference functions is derived and implemented. MRPT2 with complete active space reference functions CASPT2 is a special case. In our method the configurations with two electrons in the external orbital space are internally contracted. This ansatz strongly reduces the length of the configuration expansion as compared to uncontracted wave functions, but avoids bottlenecks occurring when fully contracted first-order wave functions are used. The method, which also allows the use of state-averaged MCSCF reference functions, is applied to optimize the structures of low-lying valence and Rydberg states of Pyrrole. Nonplanar equilibrium structures and large geometry relaxation effects on the excitation energies are found for the valence states. From the results it can be concluded that CASPT2 underestimates the excitation energies of the valence states, but leads to accurate results for the Rydberg states.}, added-at = {2019-03-01T15:49:41.000+0100}, author = {Celani, Paolo and Werner, Hans Joachim}, biburl = {https://puma.ub.uni-stuttgart.de/bibtex/29e1692bfdd9570841b80d5fd506f24fb/theochem}, doi = {10.1063/1.1597672}, interhash = {6784ab726fd842956e3d93daeb326b02}, intrahash = {9e1692bfdd9570841b80d5fd506f24fb}, issn = {00219606}, journal = {J. Chem. Phys.}, keywords = {chemie imported werner from:alexanderdenzel theoretische stuttgart theochem}, number = 10, pages = {5044–5057}, timestamp = {2019-03-01T14:49:41.000+0100}, title = {{Analytical energy gradients for internally contracted second-order multireference perturbation theory}}, url = {http://dx.doi.org/10.1063/1.1597672}, volume = 119, year = 2003 }