EXTRA_BIAS in OPES_METAD

[alx.th...@gmail.com]

Hi Plumed users and developers,

I have a general question regarding OPES_METAD. There is an option to provide EXTRA_BIAS as outlined in the manual. The example in the manual uses the save variable as driving for OPES_METAD and to put some walls restraint that is submitted to EXTRA_BIAS. However, technically it is still possible to submit any bias to this directive, computed for any variable, including ones that are quite different from the driving one. By "technically" I mean that some calculation runs, no warning or errors. But I am wondering, is this situation interpreted correctly by OPES_METAD? Is it an entirely valid setup?

Best, Alex

[Michele Invernizzi]

Hi Alex,

OPES_METAD builds its bias starting from the probability estimate along the CV obtained from reweighting, say P(s_1). Using EXTRA_BIAS, one correctly reweights the bias due to another plumed action as well. If this extra bias is a function of a different CV, say s_2,  then the reconstructed probability is no longer P(s_1), but something not well defined that is a function of s_2 as well. There might be corner cases in which this makes sense, but I can't think of one right now. If you really need this extra bias on s_2 and also need OPES_METAD to be aware of it, probably you should also bias s_2 with OPES_METAD. It makes perfect sense to use an extra bias that is a function of a subset of the CVs used in OPES_METAD.

Sometimes I found myself adding a wall potential on a secondary CV in order to avoid visiting some non-interesting metastable state (e.g. defected crystals). In this case however, you don't want to reweight for this extra bias, you want a clean FES without noise from these excluded states.

Hope this helps.

Best,

Michele

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