Active Space Embedding Theory#

Simple active space frozen-orbital embedding#

This embedding procedure provides an automatic way to embed one fragment into an environment, by an active space embedding theory that allows multireference method embedded in single-reference or multireference environment, for example, DSRG-MRPT2-in-CASSCF.

The input file should at least include two fragment:

molecule {
  0 1 # Fragment 1, system A
  ...
  --
  0 1 # Fragment 2, environment or bath B
  ...

  symmetry c1 # Currently it is suggested to disable symmetry for embedding calculations
}

In the forte options, turn on embedding procedure by adding options to forte:

set forte{
  embedding    true
  embedding_cutoff_method    threshold # threshold/cum_threshold/num_of_orbitals
  embedding_threshold    0.5 # threshold t
}

This is the minimum input required to run the embedding calculation. The embedding procedure will update the wavefunction coefficients and the MOSpaceInfo before running general forte calculations.

Four examples are available in test cases. Note that the program will by default semi-canonicalize frozen and active orbitals, if this is not intended, one can disable this semi-canonicalization with corresponding options.

EMBEDDING Options#

EMBEDDING

Turn on/off embedding procedure.

Type: bool
Default: false

EMBEDDING_CUTOFF_METHOD

The choices of embedding cutoff methods. THRESHOLD: simple threshold CUM_THRESHOLD: cumulative threshold NUM_OF_ORBITALS: fixed number of orbitals

Type: string
Options: THRESHOLD, CUM_THRESHOLD, NUM_OF_ORBITALS
Default: THRESHOLD

EMBEDDING_THRESHOLD

The threshold \(t\) of embedding cutoff. Do nothing when EMBEDDING_CUTOFF_METHOD is NUM_OF_ORBITALS

Type: double
Default: 0

EMBEDDING_REFERENCE

The reference wavefunction, do not need to specify unless using special active space treatment. Default is CASSCF with an well-defined active space including occupied and virtual orbitals.

Type: string
Default: CASSCF

EMBEDDING_SEMICANONICALIZE_ACTIVE

Turn on/off the semi-canonicalization of active space.

Type: bool
Default: true

EMBEDDING_SEMICANONICALIZE_ACTIVE

Turn on/off the semi-canonicalization of frozen core and virtual space. This will create a set of well-defined frozen orbitals.

Type: bool
Default: true

NUM_A_DOCC

The number of occupied orbitals fixed to system A, only function when EMBEDDING_CUTOFF_METHOD is NUM_OF_ORBITALS.

Type: int
Default: 0

NUM_A_UOCC

The number of virtual orbitals fixed to system A, only function when EMBEDDING_CUTOFF_METHOD is NUM_OF_ORBITALS.

Type: int
Default: 0