# 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

This is related to the derivative coupling

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.