Excitation Energy¶

(Reference: tests/CI_test/VMC-TREXIO-CH2O-excited-8724-dets-BFD-aug-cc-pVDZ)

Calculating excitation energies involves comparing the energy of the excited state to the ground state.

Strategies¶

1. Separate Energy Calculations:

   • Run VMC/DMC for the ground state.
   • Run VMC/DMC for the excited state.
   • Calculate the energy difference.

2. State-Specific Optimization:

   • Optimize the ground state wavefunction.
   • Separately optimize the excited state wavefunction.
   • Calculate the energy difference.
   • Challenge: Ensuring the excited state doesn't collapse to the ground state (variational collapse).

3. State-Average Optimization:

   • Optimize parameters to minimize the average energy of multiple states.
   • Useful when states are degenerate or close in energy.

Example (state-specific optimization)¶

%module general
    ...
    nstates       2                         # Number of states in the calculation
    weights         [ 1.0d0,  1.0d0 ]       # State weights
    weights_guiding [ 1.0d0,  1.0d0 ]       # Guiding wavefunction weights
    sr_lambda       [ 1.0d0 ]               # SR lambda
    anorm           [ 1.0d0, 0.3215d0 ]     # State amplitudes
    ...
%endmodule

%module mstates
    iguiding        2
    iefficiency     1
%endmodule