On Thu, Apr 23, 2009 at 2:54 PM, Francis E Reyes <[email protected] > wrote:
> It seems I have a case where I have 5595 reflections but my protein is > about 102 residues. With a mean atom / residue * 4 parameters for each atom > I get about 7833 parameters. So it seems that I have a observation : > parameter ratio < 1. There is only 1 molecular per asu so there's no hope in > using NCS. Phases to be obtained from a pretty good MR solution. Does > anyone have any suggestions/protocols/references for refining with > obs:parameter < 1 ? > Mathematically speaking, geometry restraints count as observations, so your ratio is probably slightly above 1. I'm not sure exactly what the formula is for figuring this out, but for each atom you have restraints on bond angles, dihedral angles, bond lengths, and VDW contacts, so it adds up to thousands. B-factors are also restrained between bonded atoms. As far as I'm aware you simply can't trust the minimization algorithms if your observation:parameter ratio is less than 1; the only option is to reduce the number of refined parameters. You may not need to do this, however. (One other comment: hydrogens will not add parameters, but they will make the geometry restraints more reliable.) -Nat
