Subsections

• • 3.13.0.1 ccsort, ccsd, and cct3 Outputs
  • 3.13.0.2 Example of a CCSD(T) calculation

3.13 CCSDT -- A Set of Coupled-Cluster Programs

The MOLCAS program CCSDT is really a shell script which calls sequentially to a set of three programs, which compute Coupled-Cluster Singles Doubles, CCSD, and Coupled-Cluster Singles Doubles and Non-iterative Triples Correction CCSD(T) wave functions for restricted single reference both closed- and open-shell systems. The set is composed by three modules: program CCSORT performs a reorganization of the integrals and the reference function from previous runs; program CCSD computes the CCSD wave function and energy allowing for different forms of spin adaptation, and program CCT3 computes the perturbative triples correction for the CCSD wave function in the different approaches explained in section  ccsdt (in users guide) of the user's guide.

There are two possibilities to run the programs. One is to use the command molcas run ccsdt $Input, where the three inputs for the three programs must be placed in file $Input. Other possibility is run the programs sequentially: molcas run ccsort, molcas run ccsd, and molcas run cct3 with their respective input files. The final possibility is to use AUTOMOLCAS. In any case the programs are run sequentially: first CCSORT, second CCSD, and, if required, CCT3.

In addition to the ONEINT and ORDINT integral files, the Comenius codes require the JOBIPH file containing the reference wave function (remember that it is not possible to compute open-shell systems with the SCF program) and the transformed two-electron integrals produced by the MOTRA module and stored in the TRAINT file.

Previously to execute the CCSORT module, wave functions and integrals have to be prepared. First, a RASSCF calculation has to be run in such a way that the resulting wave function has one single reference. In closed-shell situations this means to include all the orbitals as inactive and zero active electrons. Keyword CLOSed must be then used in the CCSORT module input. Keyword CANOnical must be used in the RASSCF input to activate the construction of canonical orbitals and the calculation of the CI-vectors on the basis of the canonical orbitals. After that the MOTRA module has to be run to transform the two-electron integrals using the molecular orbitals provided by the RASSCF module. If the LUMOrb is used in the MOTRA input it will be necessary to run a previous RASREAD program using the option CANOnical in the RASREAD input. Otherwise, the JOBIPH from the RASSCF calculation can be used directly by MOTRA using the JOBIph option in the MOTRA input. Frozen or deleted orbitals can be introduced in the transformation step by the proper options in the MOTRA input.

3.13.0.1 ccsort, ccsd, and cct3 Outputs

The section of the MOLCAS output corresponding to the CC programs is self explanatory. The default CCSORT output simply contains the wave function specifications from the previous RASSCF calculation, the orbital specifications, and the diagonal Fock matrix elements and orbital energies. The default CCSD output contains the technical description of the calculation, the iterations leading to the CCSD energy, and the five largest amplitudes of each type, which will help to evaluate the calculation. The default CCT3 output contains the description of the employed method (from the three available) to compute perturbatively the triple excited contributions to the CC energy, the value of the correction, and the energy decomposition into spin parts.

3.13.0.2 Example of a CCSD(T) calculation

Figure 3.13 contains the input files required by the seward, scf, rasscf, motra, ccsort, ccsd, and cct3 programs to compute the ground state of the HF$^+$ cation. molecule, which is a doublet of $\Sigma^+$ symmetry. A more detailed description of the different options included in the input of the programs can be found in section  ccsdt (in users guide) of the user's guide. This example describes how to calculate CCSD(T) energy for HF(+) cation. This cation can be safely represented by the single determinant as a reference function, so one can assume, that CCSD(T) method will be suitable for its description.

The calculation can be divided into few steps:

1. Run SEWARD to generate AO integrals.
2. Calculate the HF molecule at the one electron level using SCF to prepare an estimate of MO for the RASSCF run.
3. Calculate HF(+) cation by substracting one electron from the orbital with the first symmetry. There is only one electron in one active orbital so only one configuration is created. Hence, we obtain a simple single determinant ROHF reference.
4. Perform MO transfromation exploiting MOTRA using MO coeficients from the RASSCF run.
5. For any CC calculations, data produced by RASSCF and MOTRA programs need to be reorganized. This can be done using CCSORT. This code prepares all files requested by CCSD and CCT programs. All previous files can be deleted after CCSORT step.
6. Run CCSD to obtained the CCSD energy. In the prsent example T2 DDVV adaptation was chosen. Files, requested for this run are prepared by program CCSORT.
7. To obtain final CCSD(T) energy, run the CCT3 program. This code needs files, prepared by the program CCSORT and also RSTART file produced by CCSD code, in which CCSD amplitudes and energy are stored.

This is an open shell case, so it is suitable to choose CCSD(T) method as it is defined by Watts et al. [3]. Since CCSD amplitudes, produced by previous CCSD run are partly spin adapted and denominators are produced from the corresponding diagonal Fock matrix elements, final energy is sometimes refered the as SA1 ${\rm CCSD(T)_{\it d}}$ (see [4]).

A suitable shell script to run these calculations can be found at the end of section  cct3 (in users guide) of the user's guide.

Figure 3.13. Sample input containing the files required by the seward, scf, rasscf, motra, ccsort, ccsd, and cct3 programs to compute the ground state of the HF$^+$ cation.

 &SEWARD &END
Title
 HF molecule
Nopack
Symmetry
X Y
Basis set
F.ano-l...3S2P1D.
F      0.00000   0.00000   1.73300
End of basis
Basis set
H.ano-l...2S1P.
H      0.00000   0.00000   0.00000
End of basis
End of input

 &SCF &END
Title
 HF molecule
Occupied
 3 1 1 0
End of input

 &RASSCF &END
Title
 HF(+) cation
Canonical
Symmetry
 1
Spin
 2
nActEl
 1 0 0
Inactive
 2 1 1 0
Ras2
 1 0 0 0
LumOrb
End of input

 &MOTRA &END
Title
 HF(+) cation
JobIph
Frozen
 1 0 0 0
End of input

 &CCSORT &END
Title
 HF(+) cation
CCT
Frozen
1 0 0 0
End of input

 &CCSD &END
Title
  HF(+) cation
Iterations
50
Denominators
2
Shift
0.2,0.2
Accuracy
1.0d-7
Adaptation
1
Extrapolation
5,4
End of input

 &CCT3 &END
Title
 HF(+) cation
Triply
3
Denominators
0
End of input

RASSCF calculates the HF ionized state by removing one electron from the orbital in the first symmetry. Do not forget to use keyword CANONICAL.

Since this is the CCSD(T) calculation we need to prepare required files for both CCSD and CCT3 programs. Therefore CCT keyword is used.

In the CCSD run, the number of iterations is limited to 50. Denominators will be formed using orbital enegies. (This corresponds to the chosen spin adaptation.) Orbitals will be shifted by 0.2 au, what will accelerate the convergence. However, final energy will not be affected by the choosen type of denominators and orbital shifts. Required accuracy is 1.0d-7 au for the energy. T2 DDVV class of CCSD amplitudes will be spin adapted. To accelerate the convergence, DIIS procedure is exploited. It will start after 5th iteration and the last four iterations will be taken into account in each extrapolation step.

In the triples step the CCSD(T) procedure as defined by Watts et al. [3] will be performed. Corresponding denominators will be produced using diagonal Fock matrix elements.