CCL: Orbital energies for neutral and cation species in vacuum and solution

 Sent to CCL by: "Johannes Hachmann" [jh^chemistry.harvard.edu]
 Hi Scott,
 > file noted that there are now 74 beta electrons and so am I right in
 saying that
 > the first virtual beta orbital is the orbital from which the electron was
 > removed?
 Yes. BTW: Did you set up your delta-SCF as restricted open shell or
 unrestricted? Depending on the molecule you study it may be worth to compare
 the two approaches. In each case you should be consistent though, and make
 sure that you interpret any spin-polarized results carefully.
 > This orbital energy (-8.76 eV) is much lower than the highest occupied
 orbital
 > energy (-5.81 eV) in the neutral molecule.
 Interpreting KS eigenvalues - in particular for virtuals is a tricky
 business. You should check out, e.g.,
 Zhang, G.; Musgrave, C. B. Comparison of DFT Methods for Molecular Orbital
 Eigenvalue Calculations. J. Phys. Chem. A 2007, 111,
 1554-1561.
 Stowasser, R.; Hoffmann, R. What Do the KohnSham Orbitals and Eigenvalues
 Mean?. J. Am. Chem. Soc. 1999, 121, 3414-3420.
 Hamel, S.; Duffy, P.; Casida, M. E.; Salahub, D. R. Kohn Sham Orbitals and
 Orbital Energies: Fictitious Constructs but Good
 Approximations All the Same. J. Electron Spectrosc. Relat. Phenom. 2002,
 123, 345-363.
 Chong, D. P.; Gritsenko, O. V.; Baerends, E. J. Interpretation of the
 KohnSham Orbital Energies as Approximate Vertical Ionization Potentials. J.
 Chem. Phys. 2002, 116, 1760-1772.
 Luo, J.; Xue, Z. Q.; Liu, W. M.; Wu, J. L.; Yang, Z. Q. Koopmans' Theorem
 for Large Molecular Systems within Density Functional Theory. J. Phys. Chem.
 A 2006, 110, 12005-12009.
 Zhan, C.-G.; Nichols, J. A.; Dixon, D. A. Ionization Potential, Electron
 Affinity, Electronegativity, Hardness, and Electron Excitation
 Energy: Molecular Properties from Density Functional Theory Orbital
 Energies. J. Phys. Chem. A 2003, 107, 4184-4195.
 > This orbital energy (-8.76 eV) is much lower than the highest occupied
 orbital
 > energy (-5.81 eV) in the neutral molecule.
 This is not really unexpected because the Fock operator is rather different
 in the two cases. Also note the difference of occupied and virtual orbitals
 in the DFT framework.
 > So, in vacuo you have a drop in the
 > orbital energy but in solution the energy is raised. Is this a result of
 the solvent
 > having a larger effect on the energetics than the increase in positive
 charge?
 It's a bit more complicated than that. Check out this discussion:
 http://jcp.aip.org/resource/1/jcpsa6/v130/i4/p044107_s1
 I hope this helps.
 Best wishes over to Cambridge
 Johannes
 -----------------------------------------------
 Dr. Johannes Hachmann
 Research Associate
 Harvard University
 Department of Chemistry and Chemical Biology
 12 Oxford St, Rm M104A
 Cambridge, MA 02138
 -----------------------------------------------
 > -----Original Message-----
 > From: owner-chemistry+jh==chemistry.harvard.edu[]ccl.net [mailto:owner-
 > chemistry+jh==chemistry.harvard.edu[]ccl.net] On Behalf Of Scott Mckechnie
 > jsm78|,|cam.ac.uk
 > Sent: Tuesday, 14 May, 2013 09:53
 > To: Hachmann, Johannes
 > Subject: CCL: Orbital energies for neutral and cation species in vacuum
 and
 > solution
 >
 >
 > Sent to CCL by: "Scott  Mckechnie" [jsm78]-[cam.ac.uk] I am
 looking into
 the
 > change in orbital energies upon vertical ionization (i.e. ignoring nuclear
 > reorganization) for molecules in vacuum and in solution. First of all in
 vacuum, I
 > modelled the removal of an electron by performing a DFT single-point
 energy
 > calculation (on the neutral closed shell ground state gas phase optimized
 > geometry) with charge and multiplicity '12'. The single-point calculation
 output
 > file noted that there are now 74 beta electrons and so am I right in
 saying that
 > the first virtual beta orbital is the orbital from which the electron was
 > removed?
 >
 > This orbital energy (-8.76 eV) is much lower than the highest occupied
 orbital
 > energy (-5.81 eV) in the neutral molecule. However, performing the same
 > calculation in solution (with an implicit solvent model) gives a highest
 occupied
 > orbital energy of -5.65 eV for the neutral molecule and a first virtual
 beta
 > orbital energy of -5.40 eV for the cation. So, in vacuo you have a drop in
 the
 > orbital energy but in solution the energy is raised. Is this a result of
 the solvent
 > having a larger effect on the energetics than the increase in positive
 charge?
 >
 > Best wishes,
 >
 > Scott
 > jsm78.[].cam.ac.ukTo
 > recover the email address of the author of the message, please change the
 > strange characters on the top line to the [] sign. You can also look up the
 X-
 > Original-From: line in the mail header.