CCL: SV: electron affinity

 Sent to CCL by: "Frank Jensen" [frj+/-ifk.sdu.dk]
 Beware that the EA calculated by DFT in most cases is _not_ a
 well-defined quantity. For most systems the extra electron in anions is
 not bound, _if_ a sufficiently flexible basis set is used. For
 (augmented) DZ and TZ type basis sets, this shows up as positive HOMO
 energies. If the HOMO is positive, the electron is not bound, and if you
 add enough diffuse basis functions, the extra electron detaches from the
 molecule. In other words, you can get essentially any result you want by
 choosing the basis set.
 There are a few exceptions, like CN- and perhaps also F-, where the EA
 is sufficiently large that the extra electron is bound, but CO-
 certainly does not belong to this class.
 With this being said, there is a large body of computational evidence
 showing that methods like B3LYP with a DZ or TZ type basis set augmented
 with diffuse functions (like aug-cc-pVDZ or 6-311+G**) does a fairly
 good job at reproducing the experimental EA results. The reason is that
 the limited basis set constrains the extra electron to be in roughly the
 right physical space, and the functional does a reasonable job at
 estimating the energy of this confined electron density. This gives you
 a reasonable estimate for the right physical reasons, but not for the
 right theoretical reasons.
 Check your HOMO energy for the anion. If it is positive, you are relying
 on 'good' results due to error cancellations and a lucky choice of
 functional and basis set, and sometimes your luck runs out.....
 Frank
 -----Oprindelig meddelelse-----
 Fra: owner-chemistry(a)ccl.net [mailto:owner-chemistry(a)ccl.net]
 Sendt: Wednesday, November 08, 2006 2:11 PM
 Emne: CCL: electron affinity
 Sent to CCL by: "Soren Eustis" [soren/./jhu.edu]
 Your method and basis set seem more than adequate.  Electron affinity
 can
 indeed be negative.  However, in this case we know that is not true.  I
 have
 had much success calculating EA's using both DFT and MP2 - with a few
 exceptions.  A key thing to check is whether or not the multiplicity is
 input correctly.  For this case, I might see if MP2 or even HF describes
 this system better.  In fact, for this small of a molecule - one ought
 to be
 able to run a CCSD(T) calculation in a fairly short time.
 Regards,
 Soren Eustis
 -----Original Message-----
 > From: owner-chemistry:_:ccl.net [mailto:owner-chemistry:_:ccl.net]
 Sent: Wednesday, November 08, 2006 7:25 AM
 To: Eustis, Soren
 Subject: CCL: electron affinity
 Sent to CCL by: "Dr. Ponnadurai Ramasami" [ramchemi-#-intnet.mu]
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 Dear All
 I am calculating the electron affinity of carbon monoxide using =
 B3LYP/aug-cc-pVTZ by evaluating E(CO, neutral) - E(CO, negative).
 I am getting a negative value of -100.2 kJ/mol.
 The literature experimental value is 127.9 kJ/mol (I guess positive).
 Is this correct?
 Can electron affinity be negative?
 I will appreciate in case someone can help.
 Ramasami
     =20
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 <DIV><FONT face=3DArial size=3D2>Dear All</FONT></DIV>
 <DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
 <DIV><FONT face=3DArial size=3D2>I am calculating the electron
 affinity
 =
 of carbon=20
 monoxide using B3LYP/aug-cc-pVTZ by evaluating E(CO, neutral) - E(CO,=20
 negative).</FONT></DIV>
 <DIV><FONT face=3DArial size=3D2>I am getting a negative value of
 -100.2
 =
 kJ/mol.</FONT></DIV>
 <DIV><FONT face=3DArial size=3D2>The literature experimental value
 is =
 127.9 kJ/mol=20
 (I guess positive).</FONT></DIV>
 <DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
 <DIV><FONT face=3DArial size=3D2>Is this
 correct?</FONT></DIV>
 <DIV><FONT face=3DArial size=3D2>Can electron affinity be =
 negative?</FONT></DIV>
 <DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
 <DIV><FONT face=3DArial size=3D2>I will appreciate in case someone
 can=20
 help.</FONT></DIV>
 <DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
 <DIV><FONT face=3DArial size=3D2>Ramasami</FONT></DIV>
 <DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
 <DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
 <DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
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