Add efield

[brucefan1983]

To solve #602

• Usage 1:

add_efield <group_method> <group_id> <Ex> <Ey> <Ez>

This will add force $q_i \mathbf{E}$ (with components Ex, Ey, and Ez, in units of elementary charge $e$) to a group of atoms.

• Usage 2:

add_force <group_method> <group_id> <add_efield_file>

add_efield_file format:

<number_of_steps>
Ex_0 Ey_0 Ez_0
Ex_1 Ey_1 Ez_1
Ex_2 Ey_2 Ez_2
Ex_3 Ey_3 Ez_3

The electric fields in the file will be used repeatedly during the run.

• This keyword can be used more than once within a run.