From owner-chemistry@ccl.net Wed May 16 12:27:01 2007 From: "Luis M Simon luissimonrubio-#-hotmail.com" To: CCL Subject: CCL:G: Gibbs energy with solvent effects Message-Id: <-34285-070516112452-2995-lgz7wN/Ir5jVnLIpSYDduw!^!server.ccl.net> X-Original-From: "Luis M Simon" Date: Wed, 16 May 2007 11:24:49 -0400 Sent to CCL by: "Luis M Simon" [luissimonrubio*hotmail.com] If I do not misunderstood Andreas Klamt point (sorry if I am completelly wrong), the problem is that in the numerical evaluation of the hessian computational chemistry programs calculates first derivatives and energies on "perturbed" geometries, and that the cavity generated by the solvation model is perturbed aswell (as the cavity is generated from the geometry). I have experienced some problems whith frequency calculations on fully converged structures (with a solven model) that shows small negative eigenvalues in the hessian, and it makes sense to me that these problems may arrise as a "numerical noise" in the hessian evaluation. But if thats true, the problem is not the solvent model, but the neccesity of using numerical methods for calculating second derivatives. What about codes (like G03) that implements analytical second derivatives in calculations with a cavity solvation model? Will then they be more reliable? Regards: Luis Simon Sent to CCL by: Andreas Klamt [klamt(!)cosmologic.de] Hi Albert, as I already posted a few times: The vibrational contributions to solvation (at room temperature) are already implicitly aken into account in almost all solvation methods. It does not make any sense to combine them with explicit frequency calculations in the solvent, especially since the latter raise the question whther fast vibrations can be treated by equilibrium solvation. If you are going for Gibbs energies at variable temperature probably my COSMO-RS method (COSMOtherm program) is the only way to go (not just the COSMO-RS keyword in Gaussian!). Best regards Andreas Albert Poater albertpo%%stark.udg.es schrieb: Sent to CCL by: "Albert Poater" [albertpo:-:stark.udg.es] Dear Gaussian users, I have one doubt. Which are the possible methods to calculate the Gibbs energy using geometries optimized in gas phase? The calculation of frequencies, + PCM, using these geometry is the most correct one? Or taking the value for the free energy included in the PCM calculation is also valid? And correcting the gas phase Gibbs energy by the free energy of the PCM calculation minus the energy also in PCM? Hoping your comments, Albert>