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MP2 calculations with a ROHF reference state

With the program ricc2 it is possible to compute MP2 (and spin-component scaled) MP2 energies with single-determinant restricted open-shell reference wavefunctions. No additional input is required apart from the usual ROHF input for the dscf and ridft programs and a standard MPn or CC input for ricc2.

TURBOMOLEs Hartree-Fock codes can handle within the ROHF framework many cases, which include beside common high- and low-spin configuration state functions also weighted averages of high-spin CSFs (see Sec. 6.3 for further details). The Møller-Plesset perturbation theory and coupled cluster functionalities implemented in ricc2 require a single-determinant reference state and can thus only deal with high-spin open-shell cases (not averaged):

For non-abelian points groups this implies that shells with degenerate orbitals (as e.g. t1 in point group I ) must be half-filled. An average over the different (symmetry-equivalent or inequivalent) high-spin determinants that are obtained when a shell of degenerate orbitals is less than or more than half-filled is not possible with single-point ricc2 calculations.

For states with less than or more than half-filled shells of degenerate orbitals the calculations half to be done in a point group that lifts the degeneracy such that it becomes possible to assign integer occupation numbers. A symmetry-breaking of the orbitals can be avoided by doing the Hartree-Fock calculation in the full point group. The input (and MO coefficients) can then be transformed to a lower point group using define only for the ricc2 calculation.


next up previous contents index
Next: Calculations with mpgrad Up: How to Prepare and Previous: Calculations with rimp2 and   Contents   Index
TURBOMOLE M