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John Keller
Unregistered guest
Posted on Tuesday, May 05, 2015 - 6:34 pm:   

The webmo manual states that dipoles are displayed with center of the vector located at the center of mass of the molecule (or system). However, in my SO2-pyridine system (job 194346 on the demo server), I see the dipole vector centered within the pyridine ring. However, running Gaussian SP with symmetry=com, "0, 0, 0" is located between N and S, close to N. So how exactly does webmo determine the dipole position?
JR Schmidt
Username: Schmidt

Post Number: 457
Registered: 11-2006
Posted on Tuesday, May 05, 2015 - 9:28 pm:   


Typically it will place it at the COM, since almost all quantum calculations place the origin of the coordinate system at the COM. But evidently Gaussian uses the "center of atomic charge" rather than "center of mass" by default.

Note the the absolute position of the dipole is irrelevant. The dipole moment is invariant to the origin (for a neutral system).
John Keller
Unregistered guest
Posted on Wednesday, May 06, 2015 - 3:55 am:   

I am just working through various issues with different coordinate systems.
WebMO is not placing the dipole origin at the COM of this system. I have further illustrated this system with a Jmol webpage: http://tinyurl.com/m9f3rao
The table of coordinates was obtained from a Gaussian SP job with the "symmetry=com" keyword. I understand what Gaussian is doing; I don't understand what WebMO is doing.
JR Schmidt
Username: Schmidt

Post Number: 458
Registered: 11-2006
Posted on Wednesday, May 06, 2015 - 11:00 am:   


I agree. WebMO is placing the dipole at the ORIGIN, which is set by the underlying electronic structure code. In most cases this is either the COM or the center of charge.

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