# Restarting calculations¶

Restart is possible for all the SCF and coupled cluster solvers in eT.

## Global restart¶

To restart every part of a calculation, the restart keyword can be specified in the do section. The restart information will then be read from file, if eT finds the required files to restart a section. Default start guesses will be used, in case the files cannot be found. The converged $$t$$ amplitudes are used as a starting guess for the ground state multipliers. If available, the converged right excited states are used for the left excited states (and vice versa), otherwise the orbital differences are used.

To improve the convergence behavior, one might want to restart a CC3 calculation from a converged CCSD calculation. The example below shows how CC3 excitation energies are calculated by restarting from a CCSD calculation.

The minimal input to obtain two right excited states with CCSD would look like this:

system
name: H2O He
charge: 0
end system

do
excited state
end do

method
hf
ccsd
end method

solver cc es
singlet states: 2
end solver cc es

geometry
basis: cc-pVDZ
H          0.86681        0.60144        5.00000
H         -0.86681        0.60144        5.00000
O          0.00000       -0.07579        5.00000
He         0.10000       -0.02000        7.53000
end geometry


For the CC3 input the method section has to be changed from ccsd to cc3 and the global restart keyword needs to be added to the do section:

method
hf
cc3
end method

do
restart
excited state
end do


The first input will be saved as ccsd.inp and the second as cc3.inp. There are two ways to restart using the launch script. Both inputs can be handed to the launch script in the same call:

eT_launch ccsd.inp cc3.inp


At first ccsd.inp will be executed in the scratch directory, directly followed by the execution of cc3.inp in the same scratch directory. eT will find the restart information from CCSD, as the same scratch directory is used for both inputs.

Alternatively, ccsd.inp can be run first, saving its restart information using the -save flag and specifying a save directory.

eT_launch ccsd.inp -save ./


There will be a directory called ccsd_restart_files containing the restart information after the calculation has finished, because we used the submit directory as save directory. To restart CC3 from these restart files we type the following command:

eT_launch cc3.inp -load ./ccsd_restart_files/


The launch script will then copy the files from ccsd_restart_files to the scratch directory and run cc3.inp.

## Specific restart¶

The restart keyword can also be specified in the sections of the respective solvers if restart is only desired for specific parts of the calculation. The sections with a restart keyword are:

If all restart keywords are set, the effect is the same as if the global restart keyword was specified.

## Restart of multilevel coupled cluster calculations¶

For MLCC calculations, there are some special restart keywords that enter the mlcc section:

• orbital restart
• cnto restart and nto restart

The orbital restart keyword enables restart from partitioned orbitals from a previous MLCC calculation.

Note

When the orbital restart keyword is used the orbital partitioning is skipped and the orbitals on file are assumed to be suitable for the restarted calculation. The keyword is not necessary for general MLCC restart, as the orbital partitioning step will yield the same orbitals if the input for the orbitals is the same. This keyword can be used in cases when this step is computationally expensive.

If CNTOs or NTOs/canonical are requested, the cnto restart or nto restart keyword can be used to avoid the lower level CC calculation necessary to construct these orbitals: In that case, the eigenvectors of $$\boldsymbol{M}$$ and $$\boldsymbol{N}$$ (the CNTO/NTO transformation matrices) obtained in a previous MLCC calculation are read from disk.

Note

The cnto restart and nto restart keywords are only referenced if the orbital type is cnto/cnto-approx or nto-canonical, respectively.

## Restarting from eT v1.0.x¶

When running eT v1.1 (or higher), restarting from files generated with eT v1.0 requires special care. In eT v1.1, we have introduced a sign convention for Hartree-Fock orbitals and the orbitals used in MLCC calculations. Regular restart between eT 1.0.x and higher will therefore not work.

In order to restart Hartree-Fock from files generated with eT v1.0, one must run the translator script convert_v1_0_restart_files.py on the restart files from eT v1.0:

python convert_v1_0_restart_files.py /path/to/restart/files


The script can be found in the eT root folder.

When the restart files are compliant with v.1.1 and higher, the keyword skip must be used in the solver scf section in the restarted calculation:

solver scf
skip
...
end solver scf


All other sections are restarted in the normal way, with the restart keyword.

Warning

MLCC calculations cannot be restarted between v1.0 and v1.1 or older.

## Restarting from eT v1.1.x¶

When running eT v1.2 (or higher), restarting from files generated with eT v1.1.x requires special care. In eT v1.2, we have included the information of the scf_restart_file into the orbital_coefficients file.

In order to restart Hartree-Fock from files generated with eT v1.1.x, one must run the translator script convert_v1_0_restart_files.py on the restart files from eT v1.1.x, as shown above