This is part of the generic module

This action is used to align a molecule to a template.

This can be used to move the coordinates stored in plumed so as to be aligned with a provided template in PDB format. Pdb should contain also weights for alignment (see the format of PDB files used e.g. for RMSD). Make sure your PDB file is correclty formatted as explained in this page. Weights for displacement are ignored, since no displacement is computed here. Notice that all atoms (not only those in the template) are aligned. To see what effect try the DUMPATOMS directive to output the atomic positions.

Also notice that PLUMED propagate forces correctly so that you can add a bias on a CV computed after alignment. For many CVs this has no effect, but in some case the alignment can change the result. Examples are:

POSITION CV since it is affected by a rigid shift of the system.
DISTANCE CV with COMPONENTS. Since the alignment could involve a rotation (with TYPE=OPTIMAL) the actual components could be different from the original ones.
CELL components for a similar reason.

Attention: The implementation of TYPE=OPTIMAL is available but should be considered in testing phase. Please report any strange behavior.; This directive modifies the stored position at the precise moment it is executed. This means that only collective variables which are below it in the input script will see the corrected positions. As a general rule, put it at the top of the input file. Also, unless you know exactly what you are doing, leave the default stride (1), so that this action is performed at every MD step.

Warning: The molecule used for alignment should be whole. In case it is broken by the host MD code, please use WHOLEMOLECULES to reconstruct it before FIT_TO_TEMPLATE .

Compulsory keywords

STRIDE	( default=1 ) the frequency with which molecules are reassembled. Unless you are completely certain about what you are doing leave this set equal to 1!
REFERENCE	a file in pdb format containing the reference structure and the atoms involved in the CV.
TYPE	( default=SIMPLE ) the manner in which RMSD alignment is performed. Should be OPTIMAL or SIMPLE.

Examples

Align the atomic position to a template then print them

# to see the effect, one could dump the atoms before alignment
DUMPATOMS FILE=dump-before.xyz ATOMS=1-20
FIT_TO_TEMPLATE STRIDE=1 REFERENCE=ref.pdb TYPE=SIMPLE
DUMPATOMS FILE=dump-after.xyz ATOMS=1-20

(see also DUMPATOMS)