Multilevel coupled cluster calculation

Setting up an MLCC2 or MLCCSD calculation

To run an MLCC2 or MLCCSD calculation, the method mlcc2/mlccsd must be specified in the method section. E.g. for MLCC2

method
   hf
   mlcc2
end method

As multilevel methods are especially suited for intensive properties, its most relevant usage is for excited state calculations:

do
   excited state
end do

The mlcc section is necessary for every MLCC calculation, it is used to specify how the active space is generated. E.g., for MLCC2 with approximated correlated natural transition orbitals (CNTOs) tailored for the lowest excited state:

mlcc
   levels: ccs, cc2
   cc2 orbitals: cnto-approx
   cnto states: {1}
end mlcc

For MLCCSD (CCS/CCSD) with approximated CNTOs tailored for the lowest excited state:

mlcc
   levels: ccs, ccsd
   ccsd orbitals: cnto-approx
   cnto states: {1}
end mlcc

If Cholesky or Cholesky-PAO orbitals are used, an active atom space must also be defined through the active atoms section. In the example below for two waters, the first water is selected as active (atoms 1, 2, and 3)

system
   name: 2 H2O
end system

method
   hf
   mlcc2
end method

do
   excited state
end do

solver cc es
   singlet states: 2
end solver cc es

mlcc
   levels: ccs, cc2
   cc2 orbitals: cholesky-pao
end mlcc

active atoms
   selection type: range
   cc2: [1,3]
end active atoms

geometry
basis: aug-cc-pVDZ
H          0.86681        0.60144        5.00000
H         -0.86681        0.60144        5.00000
O          0.00000       -0.07579        5.00000
H          0.86681        0.60144        0.00000
H         -0.86681        0.60144        0.00000
O          0.00000       -0.07579        0.00000
end geometry

If the calculation exited successfully (look for eT terminated successfully! at the bottom of the file), a summary of the excited state calculation should be printed at the end of the mlcc2.out file

...
- Electronic excitation energies:

                                   Excitation energy
                       ------------------------------------------
   State                (Hartree)             (eV)
  ---------------------------------------------------------------
     1                  0.222134776911        6.044595162993
     2                  0.275347045324        7.492574739770
  ---------------------------------------------------------------
  eV/Hartree (CODATA 2014): 27.21138602

- Finished solving the MLCC2 excited state equations (right)
...

Relevant input sections

Active atoms

Coupled cluster excited state

Coupled cluster ground state

Multilevel coupled cluster