check calculation for solvent SLD with added components: contrast calculator

[skrueger111]

While calculating the x-ray SLD for water with 2.0M sucrose, it was noticed that the SLD is lower than reported in the literature. The same calculation using MULCh was closer to the reported value. The SLD for lower concentrations of sucrose (0.5M for instance) was much closer to that reported in the literature. MULCh requires inputting a chemical formula, molar concentration and a molecular volume. A mass density term may need to be added to the calculation, which would require a new input value. Since buffers for SANS don't typically contain components at M concentration, this issue wasn't noticed until now.

[skrueger111]

The_power_of_SANS_combined_with_deuteration_and_co.pdf

[skrueger111]

Katie Sarachan looked into this issue and discovered that both MULCh and contrast calculator give the same SLD values -- if MULCh is asked to calculate the volume of the solvent constituent (sucrose, NaCl, etc.). The element volumes that are used are from a lookup table that is common to both MULCh and contrast calculator (see screenshot below). However, if the volume of the compound is calculated from its mass density and molecular weight and then manually put into MULCh, the SAXS SLD values agree with those in the literature. Since contrast calculator uses mass density and molecular weight to calculate the SLD of solute molecules that are not protein, RNA or DNA, there is some precedent to use the same method to calculate the SLD of solvent constituents in the same manner. I suggest that we offer both volume calculations in contrast calculator as MULCh does. Users would have the choice to either let contrast calculator calculate the SLD values using the lookup table (present method) or to supply a mass density value so the the volume of the compound is calculated from the mass density. This will be addressed in a separate ticket.