Multilevel coupled cluster calculation

Currently, only ground and excited state multilevel CC2 (MLCC2) calculations can be performed with eT.

Setting up an MLCC2 calculation

To run an MLCC2 calculation, the method mlcc2 must be specified in the method section.

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 MLCC2 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

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

Save this as mlcc2.inp and type the following command in your terminal (here shown for four threads)

eT_launch --omp 4

After the calculation finished you should find mlcc2.out and mlcc2.timing.out in your working directory. Obtaining the timing file can be suppressed by specifying the -nt flag after eT_launch. 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)
...