Q-Chem 4.3 User’s Manual

6.5 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 6.4), 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[343] and is described in detail in Ref. [344]. 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 and UNRESTRICTED both to TRUE (the latter sounds as a contradiction to the method’s name, but yes, this is what you should do). 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=TRUE


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 6.123  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