This interface implements a hybrid QM/MM scheme. An subtractive approach similar to ONIOM[1] is used, this makes it possible to interface any QM and MM code without modifications.
Two implementations of the electrostatic embedding are available: 'electrostatic_2way' and 'electrostatic'.
The electrostatic_2way embedding uses point charges on the MM atoms to polarize the QM part. Reciprocally, the gradient on the point charges is calculated in the QM code. This is the correct scheme that yields exact gradient of the QM/MM energy. However, only some QM codes can produce the gradient on point charges (following interfaces are available in cuby: DFTB+, deMon, turbomole). orca).
The electrostatic embedding yields the same energy as electrostatic_2way but the gradient on the MM atoms is only approximate, based just on the MM electrostatics. Therefore, this interface is recommended only for single-point claculations, for optimizations and MD, electrostatic_2way should be used. This was the only option for electrostatic embedding in cuby3.
(The mechanical embedding can be used with any code, support for external charges is not needed.)