43.2.19 Conical Intersection optimization (CONICAL)

To optimize a conical intersection between two electronic states having the same spin, three vectors must be evaluated at SA-CPMCSCF level:

1)

Non-Adiabatic Derivative Coupling (DC).

2)

Gradient of the lower state (LSG).

3)

Gradient of the upper state (USG).

This requires three different CPMCSCF directives in the MULTI input:

CPMCSCF, NACM, $S_i$, $S_j$, ACCU=1.0d-7, record=record1.file
CPMCSCF, GRAD, $S_i$, SPIN=Spin of state $S_i$, ACCU=1.0d-7, record=record2.file
CPMCSCF, GRAD, $S_j$, SPIN=Spin of state $S_j$, ACCU=1.0d-7, record=record3.file

where $S_i$,$S_j$ are the electronic states in the usual format istate.istsym, and record[n].file specifies the name and the file number where CPMCSCF solutions should be stored. Parameter SPIN is half of the value in the WF card used to define the electronic state.

Things to remember:

i)

Specify always three different record.file on the CPMCSCF directives.

ii)

Evaluate the CPMCSCF for USG always last.

iii)

Skip the DC evaluation if the conical intersection involves states with different spin (e.g., a Singlet/Triplet crossing) because the coupling is then zero.

Three sets of FORCE commands (only two for Singlet/Triplet intersection) follow the MULTI input. They will be like:

FORCE
SAMC,record[n].file
CONICAL,record4.file[,NODC]

where record.file is one of the records containing CPMCSCF info and record4.file points to a free record used for internal storage by the CONICAL code. record4.file must be the same on all the CONICAL directives. Furthermore, the present implementation works properly only if file=1 on the CONICAL directive. The optional keyword NODC must be used in case of different spins (e.g., S/T crossing) when DC is not needed.

The actual optimization is performed using OPTG,STARTCMD=MULTI The example below optimizes the conical intersection in $LiH_2$ (ground and excited states are both doublets).

Input: lih2_D0D1.com

Output: lih2_D0D1.out

This second example optimizes the singlet-triplet intersection in $LiH_2(+)$ (ground state is Singlet, excited state is Triplet).

Input: lih2+_S0T0.com

Output: lih2+_S0T0.out