CHOOSE keyword - error when fewer arguments are used

CHOOSE keyword - error when fewer arguments are used

Postby lyuben » Mon Jul 14, 2014 10:33 am

Dear NBO Team,

I am using GenNbo6 within the ADF package and I would like to compare several different resonance structures by specifying particular bonds.
As described in the manual I use the $CHOOSE keyword to assign these bonds.
The manual also says that just few bonds/lone pairs can be listed in the $CHOOSE section (mentioned in B.8 section for NRT).
However, if I specify fewer than the total number of orbitals the calculation with GenNbo6 would fail with an error "Expecting 6 orbitals in $CHOOSE but found 4; CHOOSE terminated" (see the h2co example in the attachment) and would finish correctly with older version like GenNbo5.

Best regards,
Lyuben
Attachments
FILE47.zip
h2co example
(17.15 KiB) Downloaded 246 times
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Re: CHOOSE keyword - error when fewer arguments are used

Postby ericg » Sun Jul 20, 2014 8:28 pm

Lyuben,

Sorry about the misleading $CHOOSE description in the NBO6 manual (I'll fix this). CHOOSE must have the full specification of bonds and lone pairs. Otherwise it is unclear whether the user seeks some unusual bonding configuration or CHOOSE needs to add electron pairs.

Eric
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Re: CHOOSE keyword - error when fewer arguments are used

Postby lyuben » Mon Jul 28, 2014 4:09 am

Dear Eric,

If CHOOSE keyword (in NBO6) cannot find any more the best resonance structure given only specific bonds and/or lone pairs,
could you please explain how this can be done with NBO6.

Best regards

Lyuben
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Re: CHOOSE keyword - error when fewer arguments are used

Postby ericg » Mon Jul 28, 2014 9:34 am

Lyuben,

$CHOOSE is used to specify a particular bonding pattern for some resonance structure of interest, which may well not correspond to the best Lewis structure representation.

If you seek the best Lewis structure, then I recommend that you perform the default NBO search (i.e. don't use $CHOOSE). The default search returns the best Lewis structure fairly reliably. If you then think of alternative structures that might represent the electron distribution even better (i.e. have lower non-Lewis density), use $CHOOSE to explore these alternatives.

Eric
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Re: CHOOSE keyword - error when fewer arguments are used

Postby lyuben » Tue Jul 29, 2014 8:17 am

Dear Eric,

From your explanation I understand that the CHOOSE keyword (NBO6) now behaves as the NRT keyword (NBO6)
i.e. the functionality to find alternative resonance structures (as in NBO5 version) has been removed. Is this correct?

For example given the test I have uploaded in my first post, NBO5 would be able to properly find the positions of the lone pairs, given only the bonds no matter how they are defined.

Thank you for the reply
Lyuben
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Re: CHOOSE keyword - error when fewer arguments are used

Postby ericg » Wed Jul 30, 2014 3:14 pm

Lyuben,

Yes, I believe that you are correct. With NBO5 you could use $CHOOSE to specify only the pattern of bonds and usually expect that the search for NBOs would identify the locations of the lone pairs.

NBO6 expects that you specify the locations of both bonds and lone pairs. To aid this, NBO6 now writes a copy of the $CHOOSE keylist in the output file. This keylist can be copied into your input file and modified rather straightforwardly to reflect the bonding/lone pair pattern for any alternative structures that you would like to examine.

Eric
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