Hi everyone,

I want to use the fix spring to calculate the PMF between two large particles A , B. The A and B is, respectively, rigids with fix rigid command.

I have read the fix spring command, which says that "It can also be used to hold the centers of mass of two groups of atoms at a given distance or orientation with respect to each other. " That’s what I want.

This fix also computes global 4-vector which can be accessed by various output commands. The first 3 quantities in the vector are xyz components of the total force added to the group of atoms by the spring. In the case of the *couple* style, it is the force on the fix group (group-ID) or the negative of the force on the 2nd group (group-ID2). The 4th quantity in the vector is the magnitude of the force added by the spring, as a positive value if (r-R0) > 0 and a negative value if (r-R0) < 0. This sign convention can be useful when using the spring force to compute a potential of mean force (PMF). Good

My command is

```
fix PMF A spring couple B 100.0 NULL NULL z_value 0.0 (x_value is from 0nm to 5nm)
Then I get the f_PMF f_PMF[1] f_PMF[2] f_PMF[3] f_PMF[4]
```

My question is how to use the above five quantities to calculate the PMF. Is there a formula between the PMF and the five quantities?

Any suggestion is appreciated. Thanks in advance.