From owner-chemistry@ccl.net Fri Aug 18 12:04:00 2017 From: "Igors Mihailovs igorsm=-=cfi.lu.lv" To: CCL Subject: CCL:G: Vibrational sublevels along particular coordinate on PES Message-Id: <-52930-170818101712-13330-TETc0Zhl+ZDJeYPTyxbngw(-)server.ccl.net> X-Original-From: Igors Mihailovs Content-Language: en-US Content-Type: multipart/alternative; boundary="------------3A2C00870560F1830603987F" Date: Fri, 18 Aug 2017 17:19:21 +0300 MIME-Version: 1.0 Sent to CCL by: Igors Mihailovs [igorsm],[cfi.lu.lv] This is a multi-part message in MIME format. --------------3A2C00870560F1830603987F Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Dear computational chemistry specialists, I have a question possibly of general knowledge (which I lack), considering transfer of what I have learned about levels and sublevels of diatomics to calculations of "big" molecules. I am trying to analyze the potential energy surface of a particular compound along one particular dihedral (by doing partial optimizations at various values of this dihedral). This cut of PES has double concave shape (like the small Greek lambda), as if there were two potential wells separated by a barrier (ca. 15 kcal/mol), corresponding to two conformers. If I would be supposed to draw vibrational sublevels in both wells, what would be their energies? The computed vibrational frequencies (for stable structures at bottoms of both wells) with respect to some line over the bottom? The bottom should be the zero-point vibrational energy, but then the one computed by Gaussian is about 70 times larger than the barrier between two conformers (ca. 1015 kcal/mol). I suppose I should take only contribution to ZPE from the dihedral in interest to determine the first level in a well (since all the modes are complex, do I need some diagonalization of something, like to get "natural frequency modes"?). Is this so? If not, does this result mean there is constant interconversion of both conformers (sounds a bit ridiculous to me)? Or is this whole idea of drawing vibration levels over such a cut in PES just a nonsence? Sorry for my illiteracy. And thanks in advance! With best regards, Igors Mihailovs PhD student ISSP University of Latvia --------------3A2C00870560F1830603987F Content-Type: text/html; charset=utf-8 Content-Transfer-Encoding: 7bit Dear computational chemistry specialists,

I have a question possibly of general knowledge (which I lack), considering transfer of what I have learned about levels and sublevels of diatomics to calculations of "big" molecules.
I am trying to analyze the potential energy surface of a particular compound along one particular dihedral (by doing partial optimizations at various values of this dihedral). This cut of PES has double concave shape (like the small Greek lambda), as if there were two potential wells separated by a barrier (ca. 15 kcal/mol), corresponding to two conformers. If I would be supposed to draw vibrational sublevels in both wells, what would be their energies? The computed vibrational frequencies (for stable structures at bottoms of both wells) with respect to some line over the bottom?
The bottom should be the zero-point vibrational energy, but then the one computed by Gaussian is about 70 times larger than the barrier between two conformers (ca. 1015 kcal/mol). I suppose I should take only contribution to ZPE from the dihedral in interest to determine the first level in a well (since all the modes are complex, do I need some diagonalization of something, like to get "natural frequency modes"?). Is this so? If not, does this result mean there is constant interconversion of both conformers (sounds a bit ridiculous to me)? Or is this whole idea of drawing vibration levels over such a cut in PES just a nonsence?

Sorry for my illiteracy. And thanks in advance!

With best regards,
Igors Mihailovs
PhD student
ISSP University of Latvia --------------3A2C00870560F1830603987F--