Mathews, B. W. (1968). J. Mol. Biol. 33, 491-497
The ratio between the volume of the asymmetric unit and the molecular weight of the corresponding protein defines the Matthews coefficient VM that has dimensions of Å3Da-1 and it has usually values between 1.5 and 6 Å3Da-1.
Drenth and Mesters. Principles of protein X-Ray crystallography. Principles of protein X-Ray crystallography (2007)
"An estimation of the number of molecules per unit cell (Z) can be made by a method proposed by Matthews. He found that for many protein crystals the ratio of the unit cell volume and the molecular weight is between 1.7 and 3.5 A ̊ 3 /Dalton, with most values around 2.15 A ̊ 3 /Dalton. This number is called VM (Matthews, 1968)."
The Matthews coefficient (Vm) is calculated as:
V(unit cell) / MW * Z * X
Z is the number of asymmetric units in the unit cell (i.e. the number of symmetry operators in your space group), and the unknown variable, X, is the number of molecules in the asymmetric unit.
Advantages of a high-solvent content crystal:
If we compare low resolution (ie 3.0Å) data of a high solvent content crystal with that of a low solvent content crystal, in the former we get the same number of observations but less parameters to fit (since the model is only x% of the cell). Therefore we are better off in refinement with a high solvent content crystal. Usually crystals with high solvent content diffract badly, and they tend to benefit more of a nice intense beamline like the microfocus.