[gmx-users] least-squares fit

[Nguyen Hoang Phuong]

Dear All,

I have one question concerning to the least-squares fit in GROMACS:

suppose I have two structures obtained at the time t=0 and t=T with set of
the coordinates of atoms {r_n} and {R_n}, respectively. I have done as the
following: I fit the structure {R_n} to {r_n} and calculate the
quantities: delta_r = R'_n - {r_n} where {R'_n} are the coordinates
obtained after fitting. 

Next, I fit the structure {r_n} to {R_n} and calculate the 
quantities: delta_r' = r'_n - {R_n} where {r'_n} are the coordinates 
obtained after fitting.

The result is that delta_r differs delta_r'. According to the
least-squares fit method, the two results must be the same. Does anyone
know what the reason is? Thanks a lot in advance.

Regards,

Phuong