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Input

CASSCF Input

In this section, we construct a state-averaged CASSCF calculation using a photochromic molecular system as an example.

Unlike RHF and CIS, a CASSCF calculation requires the user to define an active space, which determines the orbitals that will participate in the multiconfigurational wavefunction. The active space is typically chosen from a previous CIS calculation, where the dominant orbital transitions reveal the chemically important orbitals.

The complete GAMESS input is shown below.

 $CONTRL RUNTYP=ENERGY SCFTYP=MCSCF MULT=1 ISPHER=1 $END

 $MCSCF CISTEP=GUGA FULLNR=.T. MAXIT=120 FORS=.T. $END

 $DRT GROUP=C1 FORS=.T. NMCC=99 NDOC=2 NVAL=2 $END

 $GUGDIA NSTATE=3 $END

 $GUGDM2 WSTATE(1)=1,1,1 $END

 $SYSTEM MWORDS=1000 MEMDDI=8000 $END

 $GUESS GUESS=MOREAD NORB=438 $END

 $BASIS GBASIS=CCD $END

 $DATA
dae-DMT-M-cf.log
C1
...
 $END

--- CLOSED SHELL ORBITALS ---
...
$VEC
...
$END

Overall Workflow

Unlike RHF or CIS, a CASSCF calculation is rarely performed as an isolated calculation. Instead, it forms part of a multistep workflow.

Geometry Optimization
        │
        ▼
RHF Calculation
        │
        ▼
CIS Calculation
        │
        ▼
Choose Active Space
        │
        ▼
State-Averaged CASSCF

The molecular orbitals generated during the RHF calculation are imported into CASSCF, while the CIS calculation helps identify the orbitals that should be included in the active space.

$CONTRL

$CONTRL
RUNTYP=ENERGY
SCFTYP=MCSCF
MULT=1
ISPHER=1
$END

This section specifies the overall type of calculation.

Keyword Description
RUNTYP=ENERGY Performs a single-point electronic energy calculation.
SCFTYP=MCSCF Requests a Multiconfigurational Self-Consistent Field calculation.
MULT=1 Specifies a singlet electronic state.
ISPHER=1 Uses spherical harmonic basis functions.

The most important keyword is

SCFTYP=MCSCF

which tells GAMESS to optimize both the molecular orbitals and the multiconfigurational wavefunction simultaneously.

$MCSCF

$MCSCF
CISTEP=GUGA
FULLNR=.T.
MAXIT=120
FORS=.T.
$END

This group controls the multiconfigurational optimization.

Keyword Description
CISTEP=GUGA Uses the Graphical Unitary Group Approach (GUGA) to generate configuration state functions.
FULLNR=.T. Uses the full Newton–Raphson algorithm for orbital optimization.
MAXIT=120 Allows up to 120 optimization cycles.
FORS=.T. Requests a Full Optimized Reaction Space calculation (Complete Active Space).

These settings are commonly used for state-averaged CASSCF calculations.

$DRT

$DRT
GROUP=C1
FORS=.T.
NMCC=99
NDOC=2
NVAL=2
$END

The Distinct Row Table (DRT) defines the electronic structure used in the CASSCF calculation.

This section specifies

  • the molecular symmetry,
  • the Complete Active Space,
  • and the orbital partitioning used during the multiconfigurational calculation.

At this stage, it is sufficient to know that

  • NMCC
  • NDOC
  • NVAL

together define the active space.

These keywords divide the molecular orbitals into

  • inactive orbitals,
  • active orbitals,
  • secondary orbitals.

The detailed meaning of these quantities will become much clearer after examining the CASSCF output, where the active orbitals, occupations, and configurations are explicitly listed.

$GUGDIA

$GUGDIA
NSTATE=3
$END

This keyword specifies the number of electronic states included in the calculation.

NSTATE=3

requests optimization of the lowest three singlet electronic states.

Optimizing multiple states simultaneously is essential when studying photochemical processes and excited-state dynamics.

$GUGDM2

$GUGDM2
WSTATE(1)=1,1,1
$END

This group defines the weighting of the electronic states.

WSTATE(1)=1,1,1

assigns equal weight to all three states.

This is known as a state-averaged CASSCF calculation.

Instead of optimizing orbitals for only one state, CASSCF determines a common set of molecular orbitals that provides a balanced description of all three electronic states.

State averaging is particularly important when studying

  • excited-state crossings,
  • conical intersections,
  • photoisomerization,
  • and nonadiabatic dynamics.

$GUESS

$GUESS
GUESS=MOREAD
NORB=438
$END

Rather than generating a new initial guess, GAMESS imports molecular orbitals from a previous calculation.

Keyword Description
GUESS=MOREAD Reads molecular orbitals from the input file.
NORB=438 Reads all 438 molecular orbitals contained in the $VEC section.

Using orbitals from a converged RHF calculation provides a reliable starting point and greatly improves the convergence of the CASSCF optimization.

$SYSTEM

$SYSTEM
MWORDS=1000
MEMDDI=8000
$END

This section controls the memory allocated to the calculation.

Keyword Description
MWORDS=1000 Allocates approximately one billion words of memory for the calculation.
MEMDDI=8000 Allocates distributed memory used during parallel execution.

Large multiconfigurational calculations typically require substantially more memory than RHF or CIS calculations.

$BASIS

$BASIS
GBASIS=CCD
$END

This section defines the atomic basis set.

The CCD basis provides a larger and more flexible description of the molecular orbitals than the minimal basis used in introductory examples, making it suitable for realistic excited-state calculations.

$VEC

The $VEC section contains the molecular orbitals obtained from the previous RHF calculation.

These orbitals serve as the starting point for the multiconfigurational optimization.

Unlike RHF, CASSCF subsequently re-optimizes these orbitals while simultaneously optimizing the configuration interaction coefficients.

Key Takeaways

  • CASSCF requires molecular orbitals from a previous RHF calculation.
  • The active space is defined through the $DRT input group.
  • Three singlet states are optimized simultaneously using state averaging.
  • Equal state weights produce a balanced description of the electronic states.
  • Both the molecular orbitals and the CI coefficients are optimized during the calculation.
  • The detailed construction of the active space becomes evident when analyzing the CASSCF output.