Volume
This keyword requests that the molecular volume be computed, defined as the volume inside a contour of 0.001 electrons/Bohr3 density. The density to be used can be specified with the Density keyword. Since a Monte-Carlo integration is done, the computed volume is only accurate to about two significant figures, but this is sufficient to estimate a radius for use with the Onsager solvent reaction field model. The recommended radius (which is 0.5 Å larger than the radius corresponding to the computed volume) is printed in the output.
Since other, more accurate solvent models are available in Gaussian 16, this keyword has applicability only in preparation for frequency calculations using SCRF=Dipole.
Tight
Requests an increased density of points for more accurate integration. By default, the volume is computed to an accuracy of about 10%. Use of this option is recommended if the computed molecular volume is needed more quantitatively.
Hartree-Fock, all DFT methods, CIS, MP2, MP3, MP4(SDQ), CID, CISD, CCD, CCSD, and QCISD.
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