Dear Sun, If we take Ian's formula for the ratio of R(free) to R(work) from his paper Acta D56 (2000) 442-450 and make some reasonable approximations, we can reformulate it as:
R(free)/R(work) = sqrt[(1+Q)/(1-Q)] with Q = 0.025pd^3(1-s) where s is the fractional solvent content, d is the resolution, p is the effective number of parameters refined per atom after allowing for the restraints applied, d^3 means d cubed and sqrt means square root. The difficult number to estimate is p. It would be 4 for an isotropic refinement without any restraints. I guess that p=1.5 might be an appropriate value for a typical protein refinement (giving an R-factor ratio of about 1.4 for s=0.6 and d=2.8). In that case, your R-factor ratio of 0.277/0.215 = 1.29 is well within the allowed range! However it should be added that this formula is almost a self-fulfilling prophesy. If we relax the geometric restraints we increase p, which then leads to a larger 'allowed' R-factor ratio! Best wishes, George Prof. George M. Sheldrick FRS Dept. Structural Chemistry, University of Goettingen, Tammannstr. 4, D37077 Goettingen, Germany Tel. +49-551-39-3021 or -3068 Fax. +49-551-39-2582
