Using BAGEL to perform quantum chemistry calculations

Units

By default, BAGEL, like Quantics, operates in atomic units. However, when specifying a molecular geometry, BAGEL provide the optional keyword angstrom (default=false: use Bohr), allowing the user to specify whether coordinates are provided in angstrom or bohr. When reading BAGEL output, Quantics assumes that the calculation was run with the default options/units ("angstrom" : false).

Non-adiabatic coupling vectors

Quantics defines the non-adiabatic coupling as

⟨φ_{i}| \frac{\partial H}{\partial R_{α}}| φ_{j}⟩

This is related to the derivative coupling

⟨φ_{i}| \frac{\partial φ_{j}}{\partial R_{α}}⟩ = ⟨φ_{i}| \frac{\partial H}{\partial R_{α}}| φ_{j}⟩ / (E_{i} - E_{j})

Within its nacme command BAGEL offers the option to calculate the full derivative coupling (nacmtype=full) or the non-adiabatic coupling (the derivative coupling without weighting by energy gap: nacmtype=noweight). Quantics assumes that BAGEL calculations of non-adiabatic couplings have been run with the latter option: "nacmtype" : "noweight".

The documentation for calculating non-adiabatic couplings in BAGEL can be found here.