# Setup for 4-point flexible water model

I tried to search online if there are examples or suggestions on how to setup a 4-point flexible water model, such as TIP4P/2005f, qTIP4P/f, TIP4P/$$\epsilon$$ FLEX, etc. but there's no clear example on how to set up intermolecular constraints, particularly for dummy atom distances and angles, with respect to the $$\ce{O}$$ and $$\ce{H}$$ atoms. I am looking for input examples for CP2K and GROMACS, but an input setup for any major classical MD software with an explanation would be really helpful.

• There is a flexible version of TIP4P in GROMACS, which can be turned on by using #define FLEXIBLE in the main topology file before reading in the water topology and parameters. However, you wish to use TIP4P/2005f, qTIP4P/f etc. for which parameter files are not shipped by default with gromacs, so I suspect that you would have to find the parameters and edit them in to make your own topology/parameter file. Aug 3 at 7:09
• @SRMaiti In the GROMACS TIP4P itp file, it is not exactly clear which version of TIP4P does the stretch and bend force constants correspond to. We can see a similar issue with SPC itp file which has different force constants than SPC/Fw itp (which is not included with default GROMACS install),
– mykd
Aug 3 at 10:29
• @SRMaiti I was about to add a bounty on this, and then realized that you had already offered one here and that it didn't bear any fruit. I don't really use GROMACS or CP2K or any of those water models, so I'm a little in the dark about this question, however do either of you have any idea why this question is so hard for the MMSE community to answer? Have either of you tried to post it on Twitter with the StackMatter tag, or to send it to any other mailing lists or facebook groups to spread the word about the bounty? Aug 24 at 0:05
• @NikeDattani I think most users on MMSE work on the side of QM/solid state etc. and there are only a few of us here who are working on MD, which is probably why it has been unanswered. I put the bounty because I have to work with flexible models in future, so this is of interest to me. I don't use facebook much, I will leave it to mykd to post it on Twitter if they wish to. Aug 24 at 3:25
• @NikeDattani Just tweeted to StackMatter. The issue is that this question has been asked on CP2K and gromacs forums almost +5 years ago, but there has been no response there.
– mykd
Aug 24 at 4:43