7.8 Coupled-Cluster Excited-State and Open-Shell Methods

7.8.20 Job Control for EOM-CC(2,3)

EOM-CC(2,3) is invoked by METHOD=EOM-CC(2,3). The following options are available:

EOM_PRECONV_SD
       Solves the EOM-CCSD equations, prints energies, then uses EOM-CCSD vectors as initial vectors in EOM-CC(2,3). Very convenient for calculations using energy additivity schemes.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       n Do n SD iterations
RECOMMENDATION:
       Turning this option on is recommended

CC_REST_AMPL
       Forces the integrals, T, and R amplitudes to be determined in the full space even though the CC_REST_OCC and CC_REST_VIR keywords are used.
TYPE:
       LOGICAL
DEFAULT:
       TRUE
OPTIONS:
       FALSE Do apply restrictions TRUE Do not apply restrictions
RECOMMENDATION:
       None

CC_REST_TRIPLES
       Restricts R3 amplitudes to the active space, i.e., one electron should be removed from the active occupied orbital and one electron should be added to the active virtual orbital.
TYPE:
       INTEGER
DEFAULT:
       1
OPTIONS:
       1 Applies the restrictions
RECOMMENDATION:
       None

CC_REST_OCC
       Sets the number of restricted occupied orbitals including frozen occupied orbitals.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       n Restrict n occupied orbitals.
RECOMMENDATION:
       None

CC_REST_VIR
       Sets the number of restricted virtual orbitals including frozen virtual orbitals.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       n Restrict n virtual orbitals.
RECOMMENDATION:
       None

To select the active space, orbitals can be reordered by specifying the new order in the $reorder_mosection. The section consists of two rows of numbers (α and β sets), starting from 1, and ending with n, where n is the number of the last orbital specified.

The following example $reorder_mo section shows orbitals 16 and 17 swapped for both α and β electrons:

$reorder_mo
   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15   17  16
   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15   17  16
$end

7.8.20.1 Examples

Example 7.71  EOM-SF(2,3) calculations of methylene.

$molecule
   0 3
   C
   H 1 CH
   H 1 CH 2 HCH

   CH  = 1.07
   HCH = 111.0
$end

$rem
   METHOD             eom-cc(2,3)
   BASIS              6-31G
   SF_STATES          [2,0,0,2]
   N_FROZEN_CORE      1
   N_FROZEN_VIRTUAL   1
   EOM_PRECONV_SD      20 Get EOM-CCSD energies first (max_iter=20).
$end

Example 7.72  This is active-space EOM-SF(2,3) calculations for methane with an elongated CC bond. HF MOs should be reordered as specified in the $reorder_mosection such that active space for triples consists of sigma and sigma* orbitals.

$molecule
   0 3
   C
   H  1  CH
   H  1  CHX  2  HCH
   H  1  CH   2  HCH  3  A120
   H  1  CH   2  HCH  4  A120

   CH   = 1.086
   HCH  = 109.4712206
   A120 = 120.
   CHX  = 1.8
$end

$rem
   METHOD            eom-cc(2,3)
   BASIS             6-31G*
   SF_STATES         [1,0]
   N_FROZEN_CORE     1
   EOM_PRECONV_SD    20   does eom-ccsd first, max_iter=20
   CC_REST_TRIPLES   1   triples are restricted to the active space only
   CC_REST_AMPL      0   ccsd and eom singles and doubles are full-space
   CC_REST_OCC       4   specifies active space
   CC_REST_VIR       17  specifies active space
   PRINT_ORBITALS    10  (number of virtuals to print)
$end

$reorder_mo
   1 2 5 4 3
   1 2 3 4 5
$end

Example 7.73  EOM-IP-CC(2,3) calculation of three lowest electronic states of water cation.

$molecule
0 1
   H   0.774767     0.000000     0.458565
   O   0.000000     0.000000    -0.114641
   H  -0.774767     0.000000     0.458565
$end

$rem
   METHOD       eom-cc(2,3)
   BASIS        6-311G
   IP_STATES    [1,0,1,1]
$end