NpT simulations for TIP4P/2005: Virtual Site Constraint/Restraint not implemented for SHAKE_ROLL!

[Christian Leitold]

Hi,

I am trying to run a NpT simulation using the TIP4P/2005 water model, which 
has an auxiliary charge site near the oxygen. To do that, I have modified 
the water_tip4p.inp input file from the tests/Fist/regtest-5 directory. NVE 
works fine, as do NVT runs, however, there is a problem when trying to run 
NpT:

"Virtual Site Constraint/Restraint not implemented for SHAKE_ROLL!"

As a matter of fact, this has been noted before:

https://groups.google.com/d/msg/cp2k/PlRK0ArPirU/R3WlZg5Yqi8J

Any idea if there es a way around that? How difficult would it be to 
actually implement the virtual site constraint for the SHAKE_ROLL 
algorithm? I know Fortran and I have implemented integrators before, but I 
assume there is a good reason why it is not implemented in the current code 
yet...

One other idea, could I instead define the "virtual" site as a regular 
particle with the proper constraints and parameters (such as zero mass)? In 
other words, what is the exact purpose of a virtual site vs. a regular one? 
I have noted that despite the TIP4P auxiliary site being defined as a 
virtual site in the input file, I still have to provide it explicitly in my 
XYZ co-ordinate input file as well. Same is true for the output XYZ, there 
the virtual site is included as well.

Thanks,
Christian
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