Q-Chem 5.1 User’s Manual

7.5 Restricted Open-Shell Kohn-Sham Method for $\Delta $-SCF Calculations of Excited States

Q-Chem provides access to certain singlet excited states – namely, those well-described by a single-electron HOMO-LUMO transition – via restricted open-shell Kohn-Sham (ROKS) theory. In contrast to the MOM approach (see Section 7.4), which requires separate SCF calculations of the non-aufbau and triplet energies, the ROKS approach attempts to combine the properties of both determinants at the level of the Fock matrix in one SCF calculation. ROKS thus presents as a single SCF loop, but the structure of the Fock matrix differs from the ground-state case. Note that this excited-state method is distinct from ROKS theory for open-shell ground states.

The implementation of ROKS excited states in Q-Chem largely follows the theoretical framework established by Filatov and Shaik[Filatov and Shaik(1999)] and is described in detail in Ref. Kowalczyk:2013. Singlet excited state energies and gradients are available, enabling single-point, geometry optimization and molecular dynamics.

To perform an ROKS excited state calculation, simply set the keywords ROKS = TRUE and UNRESTRICTED = FALSE. An additional keyword ROKS_LEVEL_SHIFT is included to assist in cases of convergence difficulties with a standard level-shift technique. It is recommended to perform a preliminary ground-state calculation on the system first, and then use the ground-state orbitals to construct the initial guess using SCF_GUESS = READ.

ROKS

Controls whether ROKS calculation will be performed.


TYPE:

LOGICAL


DEFAULT:

FALSE


OPTIONS:

FALSE

ROKS is not performed.

TRUE

ROKS will be performed.


RECOMMENDATION:

Set to TRUE if ROKS calculation is desired. You should also set UNRESTRICTED = FALSE


ROKS_LEVEL_SHIFT

Introduce a level shift of N/100 hartree to aid convergence.


TYPE:

INTEGER


DEFAULT:

0


OPTIONS:

0

No shift

N

level shift of N/100 hartree.


RECOMMENDATION:

Use in cases of problematic convergence.


Example 7.119  RO-PBE0/6-311+G* excited state gradient of formaldehyde, using the ground state orbitals as an initial guess.

$comment
   ROKS excited state gradient of formaldehyde
   Use orbitals from ground state for initial guess
$end

$rem
   EXCHANGE          pbe0
   BASIS             6-311+G*
   SCF_CONVERGENCE   9
   SYM_IGNORE        true
$end

$molecule
   0 1
   H      -0.940372    0.000000    1.268098
   H       0.940372    0.000000    1.268098
   C       0.000000    0.000000    0.682557
   O       0.000000    0.000000   -0.518752
$end

@@@

$molecule
   read
$end

$rem
   ROKS              true
   UNRESTRICTED      false
   EXCHANGE          pbe0
   BASIS             6-311+G*
   JOBTYPE           force
   SCF_CONVERGENCE   9
   SYM_IGNORE        true
   SCF_GUESS         read
$end