Restricted Open-Shell Calculations

Molecular dynamics simulations in the first excited state can be performed using Restricted Open-Shell Kohn-Sham (ROKS) theory [125]. The keyword ROKS in the &CPMD section defaults to the first excited singlet state. Solving open-shell equations is not simple unless

1. a high-spin state is computed.
2. the two singly occupied molecular orbitals (SOMOs) have different spatial symmetry.

In these two cases the Goedecker-Umrigar-Algorithm (GOEDECKER) may be used which shows the best convergence properties and is applicable in connection with Car-Parrinello molecular dynamics. Otherwise it is necessary to use a modified variant of the Goedecker-Umrigar-Algorithm and to do Born-Oppenheimer molecular dynamics (unless you know what you are doing). In almost all cases, the default algorithm (DELOCALIZED) is applicable, whereas for example some dissociation reactions require the localized variant to enable localization of the orbitals on the fragments.

      ROKS LOCALIZED

In order to make sure that the chosen algorithm works for a certain system, the conservation of energy during a molecular dynamics simulation and the shape of the orbitals should always be checked. One of the SOMOs should have the same nodal structure as the HOMO obtained by a ground state calculation. If using the unmodified Goedecker-Umrigar scheme (GOEDECKER), the energy of the singlet may collapse to approximately the triplet energy if the two SOMOs do not have different symmetries. The triplet energy can be calculated by specifying

       ROKS TRIPLET

or also

       ROKS TRIPLET GOEDECKER

See the description of the keywords LOW SPIN EXCITATION, LSE PARAMETERS and MODIFIED GOEDECKER for a description of how to do ROKS calculations using the older input LOW SPIN EXCITATION ROKS. ROKS GOEDECKER corresponds to LOW SPIN EXCITATION ROKS whereas ROKS DELOCALIZED corresponds to LOW SPIN EXCITATION ROKS with MODIFIED GOEDECKER. Do not use LOW SPIN EXCITATION in the &SYSTEM section and ROKS in the &CPMD section at the same time.

ROKS is not implemented with Vanderbilt pseudopotentials.

A Slater transition-state density between a singlet ground state and the first excited singlet state (or any pair of states described with ROKS) can be useful whenever one set of Kohn-Sham states is required which is equally well suited for each of the states involved in a transition, e.g., to calculate the couplings between the electronic transition and an external influence. This method is analogous to state-averaged multiconfigurational SCF methods and shares many of their benefits with them. In CPMD, it can be used to calculate non-adiabatic couplings between singlet states [126,127], see options COUPLINGS.