ISOR s [0.1] st [0.2] atomnamesThe named atoms are RESTRAINED with effective standard deviation s so that their Uij components approximate to isotropic behavior; however the corresponding isotropic U is free to vary. ISOR is often applied, perhaps together with SIMU, to allow anisotropic refinement of large organic molecules when the data are not adequate for unrestrained refinement of all the Uij; in particular ISOR can be applied to solvent water for which DELU and SIMU are inappropriate. ISOR should in general be applied as a weak restraint, i.e. with relatively large sigmas, for the reasons discussed above (see SIMU); however it is also useful for preventing individual atoms from becoming 'non-positive-definite'. However it should not be used indiscriminately for this purpose without investigating whether there are reasons (e.g. disorder, wrong scattering factor type etc.) for the atom going n.p.d. If (according to the connectivity table, i.e. ignoring attached hydrogens) the atom is terminal (or makes no bonds), st is used instead as the esd. If s but not st is specified, st is set to twice s. If no atoms are given, all non-hydrogen atoms are understood. SFAC element names may also be referenced, preceded by '$'. s or st may be set to zero to switch off the appropriate restraints. ISOR without atom names (or ISOR_* if residues are used) applies this restraint to all non-hydrogen atoms. Note also the use of the keyword 'LAST' to indicate the last atom in the .ins file; an anisotropic refinement of a macromolecule will often include 'ISOR 0.1 O1 > LAST', which assumes that the solvent water is in residue 0 at the end of the atom list.
Note that ISOR should in general be given a much larger esd (and hence lower weight) than DELU; whereas there is good evidence that DELU restraints should hold accurately for most covalently bonded systems, ISOR (and SIMU) are only rough approximations to reality.
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