- Search
- Download PDF

(May 16, 2021)

Singly-excited states of closed-shell molecules cannot be described via a
single non-aufbau filled Slater determinant as both the up and down spins are
equally likely to be excited, leading to at least two configurations with equal
weights. Triplet energies can nonetheless be found from a single determinant
by switching from the ${M}_{S}=0$ subspace of the ground state to ${M}_{S}=\pm 1$
(*i.e.*, by having both unpaired electrons have spins pointing in the same direction
instead of having one up and one down spin). This tactic however does not work
on singlet excited states, with non-aufbau filled configurations where only the
up (or down) spin is excited being intermediate between singlet and triplet
(and thus spin contaminated). This mixed state is not unlike spin-symmetry
broken, unrestricted ground state solutions. An actual singlet energy can be
obtained via approximate spin-purification post SCF, by removing the triplet
contribution to the energy. The triplet energy thus has to be separately
estimated with a second orbital optimization.

The *restricted open-shell Kohn-Sham* (ROKS) method offers an alternative route to singlet
excited states of this nature. The mixed non-aufbau configuration (with either
the up or down spin being excited) is exactly halfway between a singlet and
triplet when restricted open-shell orbitals are used, and has an energy
${E}_{\text{mix}}$. The triplet energy ${E}_{\text{T}}$ is also computable from a single
determinant within the the ${M}_{S}=\pm 1$ subspaces. Consequently, ROKS optimizes
a set of spin-restricted orbitals $\{{\varphi}_{\text{ROKS}}\}$ such that the spin-purified
singlet energy ${E}_{\text{S}}=2{E}_{\text{mix}}[\{{\varphi}_{\text{ROKS}}\}]-{E}_{\text{T}}[\{{\varphi}_{\text{ROKS}}\}]$ is
stationary. This therefore needs only one orbital optimization, in contrast to
the two sets needed for the $\mathrm{\Delta}$SCF approach mentioned in the preceding
paragraph. The structure of the ROKS Fock matrix however is more complex,
^{
572
}
J. Chem. Phys.

(2013),
138,
pp. 164101.
Link
by virtue of the two-determinant nature of the equations.
It is also important to note that this excited-state method is
distinct from ROKS theory for open-shell ground states, which is a single-determinant method
corresponding to the high-spin state with multiple unpaired spins.

The implementation of ROKS excited states in Q-Chem largely follows the
theoretical framework established by Filatov and Shaik;
^{
312
}
Chem. Phys. Lett.

(1999),
304,
pp. 429.
Link
see Ref. 572 for the case of the
lowest excited singlet (${S}_{1}$ state) with a DIIS-based approach. An example is provided below. ROKS for higher excited states is possible using either the squared-gradient approach
(Section 7.8.2) or else state-targeted energy projection (Section 7.8.3).

ROKS has been found to be significantly more accurate than TDDFT for describing
charge-transfer states,
^{
400
}
J. Chem. Theory Comput.

(2016),
12,
pp. 3353.
Link
and preliminary evidence
shows the same to hold for Rydberg states. ROKS is also extremely accurate for
core excitation energies, with the SCAN functional yielding errors below 0.5 eV for both K- and L-edge excitations
of small molecules.
^{
397
}
J. Phys. Chem. Lett.

(2020),
11,
pp. 775.
Link
Examples of using ROKS/SGM to compute core-excited states are provided in
Section 7.8.2. Analytic nuclear gradients (in the excited state) are
also available, enabling geometry optimization and molecular dynamics
calculations as well, along with finite-difference frequency calculations.
Users of the ROKS code are requested to cite Ref. 572, and in addition
Ref. 398 if the SGM implementation is
employed, as well as Ref. 400 for charge-transfer states and
Ref. 397 for application to core excitations.

The chief limitations of ROKS are that it can only describe states with one broken electron pair. It is consequently applicable only to certain excited states of closed-shell systems: all singlet single excitations well-described by a single natural transition orbital (NTO) pair, or higher singlets where only one electron pair is broken in total (like the ${}^{1}$B${}_{3g}$ doubly excited state of tetrazine). Fortunately, most charge-transfer and core-excitations do not require more than one broken electron pair, and so this limitation is not a major problem in practice.

To perform an ROKS excited state calculation, simply set the keyword ROKS = TRUE and ensure that UNRESTRICTED = FALSE. An additional keyword ROKS_LEVEL_SHIFT is included to assist in cases of DIIS 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. Make sure that
UNRESTRICTED is not set to TRUE.

ROKS_LEVEL_SHIFT

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

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0
No shift
$N$
level shift of N/100 hartree.

RECOMMENDATION:

Use in cases of problematic DIIS convergence.

$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