I am trying to use Chimera to list percent accessible surface area by residue for a structure by calculating (solvent accessible area SAS) / (solvent excluded area, SES). For some reason, for residues that are completely buried, Chimera returns not only SAS=0, but also SES=0, and thus those residues aren't included in the percent accessible calculation. Does anyone have a clue for this behavior?

My workflow is as follows:

File>Open  [choose pdb file]
Actions>Surface>Show
Tools>Surface/Binding Analysis>Measure Volume and Area
Tools>Depiction>Render by Attribute

in the render by attribute window:

File>Save Attributes

in the save window:

select attribute 'areaSAS' or 'areaSES' of 'residues' and save to a file

to calculate % accessible:
Tools>Structure Analysis>Attribute Calculator

in the attribute calculator window:
Calculate attribute "percentaccessible" for "residues"
Formula:
(residue.areaSAS)/(residue.areaSAS+residue.areaSES)
[check Save calculation results to file]