Introduction

A useful example of how to use the LMTRANS, LMFIT and LMPLOT programs is provided by the NH3 molecule. Specifically, we consider in the following the example of the dipole surfaces for the first two electronic states of NH3 expanded about the S1 state D3h minimum geometry along the inversion mode Q1.

Using LMTRANS

The LMTRANS program is first used to transform the calculated adiabatic dipole matrix to the (quasi) diabatic representation. To do so, the input file nh3_d3h_q1_dip.inp is used. The normal modes are read from the Gaussian output file nh3_d3h_freq.log, and the names of Molpro output files containing the adiabatic dipole matrix calculated at the EOM-CCSD/aug-cc-pVDZ level at points along the invedrsion coordinate Q1 are specified in the file q1_dip.set. Notice the use of the negphase keyword to flag when a change in phase causes the transition dipole moments between states 1 and 2 to change sign. The rotation of the dipole matrix to the (quasi) diabatic representation is achieved using the eigenvectors of a vibronic coupling Hamiltonian written to the file nh3_d3h_q1_fit.vcham using the VCHFIT program. Running the lmtrans program using the command

lmtrans nh3_d3h_q1_dip.inp

results in the creation of the database file nh3_d3h_q1_dip.dat. This file contains for each file specified in the file q1_dip.set nuclear geometry in both Cartesian coordinates and normal modes, and the dipole matrix in both the adiabatic and (quasi) diabatic representations. This database can now be used in a subsequent fitting procedure using the LMFIT program.


Using LMFIT

Using the database file nh3_d3h_q1_dip.dat, fits to the elements of the (quasi) diabatic dipole matrix along the mode inversion mode Q1 are made using the LMFIT program. The input file nh3_d3h_q1_lmfit.inp is used. The LMFIT program can only handle Abelian point groups. Hence, the highest subgroup of the D3 point group of the reference geometry is used, that is, the C2v point group. Note that attention has to be paid to ensuring that the state and axis symmetries are correct for the orientation of the molecule at the reference geometry. Running the lmfit program using the command

lmfit nh3_d3h_q1_lmfit.inp

results in the creation of the file nh3_d3h_q1_lmfit.lmi, which contains the values of the fitted parameters of the Taylor expansions of the (quasi) diabatic dipole matrix elements.


Using LMPLOT

Using the LMPLOT program, the model dipole matrix elements contained in the file nh3_d3h_q1_lmfit.lmi and/or the calculated dipole matrix elements contained in the database file nh3_d3h_q1_dip.dat may be visualised. For example, to plot the z-component of the (quasi) diabatic transition dipole moment between the states 1 and 2 along the mode Q1, the following command woud be used:

lmplot -dipole z 1 2 nh3_d3h_q1_lmfit 1