Geometry Optimization and Vibrational Frequency Analysis

The above example was a simple single-point energy computation. To perform a different type of computation, change the keyword jobtype. In the example below, we will set up a CCSD geometry optimization. To illustrate a more flexible z-matrix input, we will now define variables for the bond length and bond angle (in the zvars section).

% 6-31G** H2O Test optimization calculation

psi: (
  label = "6-31G** SCF H2O"
  jobtype = opt
  wfn = ccsd
  reference = rhf
  dertype = first
  basis = "6-31G**"
  zmat = (
    o
    h 1 roh
    h 1 roh 2 ahoh
  )
  zvars = (
    roh     0.96031231
    ahoh  104.09437511
  )
)

Once you have optimized the geometry of a molecule, you might wish to perform a frequency analysis to determine the nature of the stationary point. To do this, change the value of jobtype to freq. For an SCF frequeny calculation, you would also set dertype = second to compute the second derivatives analytically. Unfortunately, analytical second derivitives are not available in PSI3 for wavefunctions beyond SCF, so instead use the highest order analytical derivitives that are available for the type of wavefunction you have chosen. This information is given in Table 2. For our CCSD example, the highest-order derivitives available are first, so dertype = first.

% 6-31G** H2O Test computation of frequencies

psi: (
  label = "6-31G** SCF H2O"
  jobtype = freq
  wfn = ccsd
  reference = rhf
  dertype = first
  basis = "6-31G**"
  zmat = (
    o
    h 1 roh
    h 1 roh 2 ahoh
  )
  zvars = (
    roh     0.96031231
    ahoh  104.09437511
  )
)