This is part of the vatom module

Add a virtual atom in a fixed position.

This action creates a virtual atom at a fixed position. The coordinates can be specified in Cartesian components (by default) or in scaled coordinates (SCALED_COMPONENTS). It is also possible to assign a predefined charge or mass to the atom.

Attention: Similar to POSITION this variable is not invariant for translation of the system. Adding a force on it can create serious troubles.

Notice that the distance between two atoms created using FIXEDATOM is invariant for translation. Additionally, if one first align atoms to a reference using FIT_TO_TEMPLATE, then it is safe to add further fixed atoms without breaking translational invariance.

Examples

The following input instructs plumed to compute the angle between distance of atoms 15 and 20 and the z axis and keeping it close to zero.

Click on the labels of the actions for more information on what each action computes

a: FIXEDATOM ATcompulsory keyword 
coordinates of the virtual atom =0,0,0 The FIXEDATOM action with label a calculates the position of a virtual atom
b: FIXEDATOM ATcompulsory keyword 
coordinates of the virtual atom =0,0,1 The FIXEDATOM action with label b calculates the position of a virtual atom
an: ANGLE ATOMSthe list of atoms involved in this collective variable (either 3 or 4 atoms). =a,b,15,20 The ANGLE action with label an calculates a single scalar value
RESTRAINT ARGthe input for this action is the scalar output from one or more other actions. =an ATcompulsory keyword 
the position of the restraint =0.0 KAPPAcompulsory keyword ( default=0.0 )
specifies that the restraint is harmonic and what the values of the force constants
on each of the variables are =100.0 The RESTRAINT action with label

The following input instructs plumed to align a protein to a template and to then compute the distance between one of the atoms in the protein and the point (10,20,30).

Click on the labels of the actions for more information on what each action computes

FIT_TO_TEMPLATE STRIDEcompulsory keyword ( default=1 )
the frequency with which molecules are reassembled. =1 REFERENCEcompulsory keyword 
a file in pdb format containing the reference structure and the atoms involved in
the CV. =ref.pdb TYPEcompulsory keyword ( default=SIMPLE )
the manner in which RMSD alignment is performed. =SIMPLE The FIT_TO_TEMPLATE action with label 
a: FIXEDATOM ATcompulsory keyword 
coordinates of the virtual atom =10,20,30 The FIXEDATOM action with label a calculates the position of a virtual atom
d: DISTANCE ATOMSthe pair of atom that we are calculating the distance between. =a,20 The DISTANCE action with label d calculates a single scalar value
PRINT ARGthe input for this action is the scalar output from one or more other actions. =d FILEthe name of the file on which to output these quantities =colvar The PRINT action with label

The reference structure to align to is provided in a pdb file called ref.pdb as shown below:

ATOM      8  HT3 ALA     2      -1.480  -1.560   1.212  1.00  1.00      DIA  H
ATOM      9  CAY ALA     2      -0.096   2.144  -0.669  1.00  1.00      DIA  C
ATOM     10  HY1 ALA     2       0.871   2.385  -0.588  1.00  1.00      DIA  H
ATOM     12  HY3 ALA     2      -0.520   2.679  -1.400  1.00  1.00      DIA  H
ATOM     14  OY  ALA     2      -1.139   0.931  -0.973  1.00  1.00      DIA  O
END

Glossary of keywords and components

The atoms involved can be specified using

ATOMS

the list of atoms which are involved the virtual atom's definition. For more information on how to specify lists of atoms see Groups and Virtual Atoms

Compulsory keywords

AT	coordinates of the virtual atom
SET_MASS	( default=1 ) mass of the virtual atom
SET_CHARGE	( default=0 ) charge of the virtual atom

Options

SCALED_COMPONENTS

( default=off ) use scaled components