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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):

- The spins of all
*n*_{open}
unpaired electrons are parallel (*α*
spin will be assumed)
so that the ROHF reference state has the spin multiplicity
*n*_{open} + 1
.
- There must be only one type of open shells and all orbitals in this shell must
have the occupation number 1.
- For the single electron case (i.e. doublets) the Roothaan parameters
are
*a* = *b* = 0
, for high-spin cases with more than one unpaired electron
the Roothaan parameters must be set to *a* = 1
and *b* = 2
.

For non-abelian points groups this implies that shells with degenerate orbitals
(as e.g. *t*_{1}
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.

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** Up:** How to Prepare and
** Previous:** Calculations with rimp2 and
** Contents**
** Index**
TURBOMOLE M