This is part of the colvar module

Calculate the FRET efficiency between a pair of atoms. The efficiency is calculated using the Forster relation:

\[ E=\frac{1}{1+(R/R_0)^6} \]

where \(R\) is the distance and \(R_0\) is the Forster radius.

By default the distance is computed taking into account periodic boundary conditions. This behavior can be changed with the NOPBC flag.

The atoms involved can be specified using

ATOMS

the pair of atom that we are calculating the distance between. For more information on how to specify lists of atoms see Groups and Virtual Atoms

Compulsory keywords

R0	The value of the Forster radius.

Options

NUMERICAL_DERIVATIVES

( default=off ) calculate the derivatives for these quantities numerically

NOPBC

( default=off ) ignore the periodic boundary conditions when calculating distances

Examples

The following input tells plumed to print the FRET efficiencies calculated as a function of the distance between atoms 3 and 5 and the distance between atoms 2 and 4.

fe1:  FRET ATOMS=3,5 R0=5.5
fe2:  FRET ATOMS=2,4 R0=5.5
PRINT ARG=fe1,fe2

(See also PRINT).

The following input computes the FRET efficiency calculated on the terminal atoms of a polymer of 100 atoms and keeps it at a value around 0.5.

WHOLEMOLECULES ENTITY0=1-100
fe: FRET ATOMS=1,100 R0=5.5 NOPBC
RESTRAINT ARG=fe KAPPA=100 AT=0.5

(See also WHOLEMOLECULES and RESTRAINT).

Notice that NOPBC is used to be sure that if the distance is larger than half the simulation box the distance is compute properly. Also notice that, since many MD codes break molecules across cell boundary, it might be necessary to use the WHOLEMOLECULES keyword (also notice that it should be before FRET). Just be sure that the ordered list provide to WHOLEMOLECULES has the following properties:

Consecutive atoms should be closer than half-cell throughout the entire simulation.
Atoms required later for the distance (e.g. 1 and 100) should be included in the list