David: The "solid's pressure" is minus the derivative of total energy with respect to volume. More precisely, in siesta, with respect to a uniform scaling of all unit cell vectors and coordinates.
For a system of well separated molecules, this definition is nonintuitive, because it includes an intramolecular pressure due to the scaling of the intramolecular coordinates: P_intramol = sum_i (f_i * r_i) / Cell_volume The sum is called the virial and it is subtracted in siesta to find a "molecular-system's pressure" which should be zero for molecular systems with well converged grids and basis sets. Of course, the virial cancels for a well relaxed geometry, because then the forces are zero. Best wishes, Jose M. Soler On Fri, Jun 5, 2009 at 8:03 PM, David Strubbe<[email protected]> wrote: > Hello, > > I asked this a little while ago and didn't get an answer. Can anyone tell > me what the difference is between "solid" and "molecule" pressure in the > output? > > David Strubbe > > ---------- Forwarded message ---------- > From: David Strubbe <[email protected]> > Date: Tue, May 12, 2009 at 4:28 PM > Subject: solid vs molecule pressure > To: "Siesta, Self-Consistent DFT LCAO program, http://www.uam.es/siesta"; > <[email protected]> > > > Hello SIESTA users, > > I noticed that the SIESTA 2.0.1 output offers two values for the pressure: > "solid" and "molecule." In both my bulk and molecule calculations the two > are very similar. What is the difference in how they are calculated? The > manual seems not to make any mention of this distinction. > > David Strubbe > UC Berkeley > >
