Closed shell CCSD

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Running CCSD(T) calculations is straightforward if the ground state of the molecule can be well-described by a single closed shell determinant. This is the case for many molecules for which the input requires specification of the basis set (TZ or better is recommended, but for this example we will take DZ to reduce the run time). We take the inter halogen molecule ClF as an example and use the default cut-offs for correlating electrons (include all valence electrons with energy above -10 Hartree) and truncation of virtual space (delete virtuals above 20 Hartree). The geometry (clf.xyz) file is very simple, requiring only specification of the coordinates. The program will then identity the symmetry as C_{\inf v}: 

2
ClF molecule at equilibrium distance taken from NIST.
Cl   0.0                 0.0                 0.0            
F    0.0                 0.0                 1.628

We specify the wave function type (SCF, followed by RELCCSD) and basis set in the input file (cc.inp) to calculate the CCSD(T) energy with the Dirac-Coulomb Hamiltonian (with the contribution from SS|SS integrals approximated by a simple Coulombic correction). 

**DIRAC
.WAVE FUNCTION
**WAVE FUNCTION
.SCF
.RELCCSD
**MOLECULE
*BASIS
.DEFAULT
cc-pVDZ
*END OF INPUT


Now start the calculation by executing: 

pam  --mol=clf.xyz --inp=cc.inp

If everything works fine the results of the calculation will be written to cc_clf.out. In addition an archive file cc_clf.tgz will be created. Depending on the type of calculation the archive file may include:

  • DFCOEF containing molecular orbital coefficients and possibly energies.
  • *.cube containing plots of molecular properties, typically electron densities, in the gridded Gaussian Cube format.
  • Additional files for restarting particular types of calculations.
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