MOLPRO

25 EXCITED STATES WITH EQUATION-OF-MOTION CCSD (EOM-CCSD) 188
25 EXCITED STATES WITH EQUATION-OF-MOTION CCSD
(EOM-CCSD)
Excitation energies for singlet states can be computed using equation-of-motion (EOM) approach.
For the excitation energies the EOM-CCSD method gives the same results as linear
response CCSD (LR-CCSD) theory. Accurate results can only be expected for singly excited
states. The states to be computed are specified on an EOM input card, which is a subcommand
of CCSD. The following input forms are possible
EOM, state1, state2, state3, . . .
Computes the given states. Each state is specified in the form number.sym, e.g., 5.3 means the
fifth state in symmetry 3. Note that state 1.1 corresponds to the ground state CCSD wavefunction
and is ignored if given.
EOM, −n1.sym1, −n2,sym2, . . .
computes the first n1 states in symmetry sym1, n2 in sym2 etc.
EOM, n1.sym1, −n2,sym1, . . .
computes states n1 through n2 in symmetry sym1.
The different forms can be combined, e.g.,
EOM, −3.1, 2.2, 2.3, −5.3
computes states 1-3 in symmetry 1, the second excited state in symmetry 2, and the second
through fifth excited states in symmetry 3. Note that state 1.1 is the ground-state CCSD wavefunction.
By default, an error exit will result if the CCSD did not converge and a subsequent EOM calculation
is attempted. The error exit can be avoided using the NOCHECK option on the CCSD
command (see also CCSD(T)).
25.1 Options for EOM
Normally, no further input is needed for the calculation of excitation energies.
EOM-CCSD amplitudes can be saved using SAVE=record.ifil. The vectors will be saved after
every refreshing of the iteration space and at the end of the calculation. The calculation can
be restarted from the saved vectors, if START=record.ifil is specified. The set of vectors to be
computed can be different in old and restarted calculations. However, if both cards (SAVE and
START) are specified and the records for saving and restarting are identical, the sets of vectors
should be also identical, otherwise chaos. The identical SAVE and START records can be useful
for potential energy surfaces calculations, see section 25.4.1.
By default, only excitation energies are calculated, since the calculation of properties is about
two times as expensive, as the calculation of energies only. The one-electron properties and
transition moments (expectation type, as defined in: J.F. Stanton and R.J. Bartlett, J. Chem.
Phys., 98 7029 (1993)) can be calculated by adding TRANS=1 to EOM card. The CCSD ground
state is treated as a special case. If RELAX option is specified on the EXPEC card, also the
relaxed one-electron density matrix is calculated for the ground state. (Currently, the relaxed
CCSD density matrix is available for all-electron calculations only.) By default, dipole moments
are calculated. Other required properties can be specified using EXPEC card. The excited state
densities are saved if a DM card is present. For an example see section 25.4.2. If properties are
calculated, they are saved in MOLPRO variables, e.g. the x-component of the dipole moment is