Input¶
Input File¶
The CIS input is almost identical to an RHF calculation. The only changes are the keywords required to perform an excited-state calculation and to read the converged Hartree–Fock molecular orbitals.
The complete input is shown below.
$CONTRL COORD=CART UNITS=ANGS RUNTYP=ENERGY SCFTYP=RHF CITYP=CIS $END
$BASIS GBASIS=STO NGAUSS=3 $END
$SYSTEM MWORDS=200 $END
$GUESS GUESS=MOREAD NORB=7 $END
$CIS NSTATE=3 MULT=1 $END
$DATA
h2o.log
C1
O 8.0 0.0000000000 0.0000000000 0.1068300000
H 1.0 0.0000000000 0.7851780000 -0.4273190000
H 1.0 0.0000000000 -0.7851780000 -0.4273190000
$END
... RHF molecular orbitals ...
$VEC
...
$END
The following changes can be seen compared to an RHF calculation.
CITYP=CIS¶
The CITYP keyword tells GAMESS which Configuration Interaction method should be used.
Setting
requests a Configuration Interaction Singles calculation.
GAMESS first reads the converged RHF molecular orbitals and then generates all possible single excitations from the occupied orbitals to the virtual orbitals.
$CIS Group¶
The $CIS group controls the excited-state calculation.
NSTATE¶
Specifies the number of excited states to calculate.
For this example,
requests the lowest three excited states.
Increasing this number instructs GAMESS to compute more excited states.
MULT¶
Specifies the spin multiplicity of the excited states.
| Value | Excited State |
|---|---|
1 |
Singlet |
3 |
Triplet |
Since water has a closed-shell singlet ground state, this example computes the first three singlet excited states.
$GUESS¶
Unlike an RHF calculation, CIS does not generate new molecular orbitals.
Instead, it uses the converged RHF orbitals from a previous calculation.
The dat file of RHF calculation consist of the closed shell orbitals to be used for CIS calculation.
GUESS=MOREAD¶
tells GAMESS to read molecular orbitals supplied in the $VEC group.
Without these orbitals, a CIS calculation cannot proceed.
NORB¶
Specifies the number of molecular orbitals that should be read from the $VEC section.
For this water molecule,
indicates that seven molecular orbitals are available from the RHF calculation.
This Number can also be obtained by the following search in RHF log file
$VEC¶
The $VEC section contains the molecular orbital coefficients obtained from the previous RHF calculation.
These orbitals define the Hartree–Fock reference wavefunction and serve as the starting point for generating singly excited electronic configurations.
The $VEC group is copied directly from the RHF output and should not be modified manually unless the orbitals themselves are changed.
Note
Every CIS calculation requires a valid Hartree–Fock reference wavefunction. In GAMESS, this is typically supplied through the $VEC group using GUESS=MOREAD.