14 SCF Analytic Force Constants for Ethylene

We consider below computing the analytic force constants for C₂H₄, initially optimising the molecule at the SCF level, followed by the force constant calculation. Note the use of the XTOL directive in the optimisation job to ensure a higher degree of optimisation than that derived using the default XTOL.

Geometry Optimisation

          #!/bin/csh -f
          cd /scr1/user
          setenv ed2 c2h4main
          setenv ed3 c2h4dump
          /scr1/wab/GAMESS-UK/bin/gamess << EOF
          title 
          ethylene 6-31g**  geometry optimisation
          zmatrix angstrom
          c
          c 1 cc
          h 1 ch 2 hcc
          h 1 ch 2 hcc 3 180.0
          h 2 ch 1 hcc 3 0.0
          h 2 ch 1 hcc 3 180.0
          variables
          cc 1.40
          ch 1.10
          hcc 118.0
          end
          basis 6-31g**
          runtype optimize
          xtol 0.0001
          enter
          EOF

Analytic Force Constants

          #!/bin/csh -f
          cd /scr1/user
          setenv ed2 c2h4main
          setenv ed3 c2h4dump
          /scr1/wab/GAMESS-UK/bin/gamess << EOF
          restart
          title 
          ethylene 6-31g** ground state vibrational frequencies
          zmatrix angstrom
          c
          c 1 cc
          h 1 ch 2 hcc
          h 1 ch 2 hcc 3 180.0
          h 2 ch 1 hcc 3 0.0
          h 2 ch 1 hcc 3 180.0
          variables
          cc 1.40
          ch 1.10
          hcc 118.0
          end
          basis 6-31g**
          runtype hessian
          enter
          EOF