This is part of the colvar module

Calculate Debye-Huckel interaction energy among GROUPA and GROUPB.

This variable calculates the electrostatic interaction among GROUPA and GROUPB using a Debye-Huckel approximation defined as

\[ \frac{1}{4\pi\epsilon_r\epsilon_0} \sum_{i\in A} \sum_{j \in B} q_i q_j \frac{e^{-\kappa |{\bf r}_{ij}|}}{|{\bf r}_{ij}|} \]

This collective variable can be used to analyze or induce electrostatically driven reactions [28]. Notice that the value of the DHENERGY is returned in plumed units (see UNITS).

If GROUPB is empty, it will sum the N*(N-1)/2 pairs in GROUPA. This avoids computing twice permuted indexes (e.g. pair (i,j) and (j,i)) thus running at twice the speed.

Notice that if there are common atoms between GROUPA and GROUPB their interaction is discarded.

The atoms involved can be specified using
GROUPA First list of atoms. For more information on how to specify lists of atoms see Groups and Virtual Atoms
GROUPB Second list of atoms (if empty, N*(N-1)/2 pairs in GROUPA are counted). For more information on how to specify lists of atoms see Groups and Virtual Atoms
Compulsory keywords
I ( default=1.0 ) Ionic strength (M)
TEMP ( default=300.0 ) Simulation temperature (K)
EPSILON ( default=80.0 ) Dielectric constant of solvent
NUMERICAL_DERIVATIVES ( default=off ) calculate the derivatives for these quantities numerically
NOPBC ( default=off ) ignore the periodic boundary conditions when calculating distances
SERIAL ( default=off ) Perform the calculation in serial - for debug purpose
PAIR ( default=off ) Pair only 1st element of the 1st group with 1st element in the second, etc

( default=off ) Use a neighbour list to speed up the calculation

NL_CUTOFF The cutoff for the neighbour list

The frequency with which we are updating the atoms in the neighbour list

# this is printing the electrostatic interaction between two groups of atoms
dh: DHENERGY GROUPA=1-10 GROUPB=11-20 EPSILON=80.0 I=0.1 TEMP=300.0

(see also PRINT)