From chemistry-request@server.ccl.net Thu Mar 6 11:41:48 2003 Received: from evans.univer.kharkov.ua ([80.92.225.132]) by server.ccl.net (8.11.6/8.11.0) with ESMTP id h26Geca19608 for ; Thu, 6 Mar 2003 11:40:41 -0500 Received: from spin.univer.kharkov.ua (spin.univer.kharkov.ua [192.168.213.252]) by evans.univer.kharkov.ua (8.12.6/8.12.5) with SMTP id h26GeAfS029511; Thu, 6 Mar 2003 18:40:15 +0200 Received: by spin.univer.kharkov.ua with Microsoft Mail id <01C2E40F.3D79F3C0@spin.univer.kharkov.ua>; Thu, 6 Mar 2003 18:36:02 +0200 Message-ID: <01C2E40F.3D79F3C0@spin.univer.kharkov.ua> From: "sergey i. kotelevskii" To: "'p smith'" , "'CCL community'" Subject: RE: infra-red stretching frequencies Date: Thu, 6 Mar 2003 18:36:01 +0200 MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by server.ccl.net id h26Gfma19629 ---------- From: p smith Sent: Friday, February 28, 2003 3:10 AM To: chemistry@ccl.net Subject: CCL:infra-red stretching frequencies Hello, It depends. That is, the cheapest/reliable method depends on the system under study. For qualitative purposes on large stable systems, you may use AM1 or PM3 frequencies scaling them heavily above ~ 1500-2000 cm(-1). However, you couldn't expect reliability e.g. from Hydrogen-bonded complexes in low-frequency region. As the cheapest universal tool, DFT B3LYP is recommended, with mandatory polarization functions on heavy atoms (and hydrogens for X-H stretch). Also take a look at: A.P.Scott and L. Radom, "Harmonic Vibrational Frequencies: An Evaluation of Hartree-Fock, Moller-Plesset, Quadratic Configurational Interaction, Density Functional Theory, and Semiempirical Scale Factors", J. Phys. Chem. 1996, V.100, 16502-16513. Best regards, Serghey. **************************************************************************************** Serghey I. Kotelevskii, Department of Theoretical Chemistry and Astrochemistry, Research Institute for Chemistry, Kharkiv VNKarazin National University, Svobody Square 4, UA-61077 Kharkiv, Ukraine Hi, What is the cheapest method of producing reliable IR stretching frequencies computationally i.e. which represents the best compromise between cost and accuracy. I know these calculations can be performed semi-empirically but I am not sure how accurate the results are. Cheers, Paul p.smith@student.umist.ac.uk