# [molpro-user] How to define occ, closed and core n1, n2, n3, n4, n5, n6, n7, n8?

Neeraj Rai neerajrai at gmail.com
Thu Jun 2 20:22:30 BST 2011

```Wenli,

Not long ago [may be even now!] I was in similar situation as you find
yourself in. Now if you want to use any program be it gaussian or molpro
efficiently/correctly, you will need to learn the background information
because there are lot of things that will trip you from time to time. Just
because one program lets you set up the simulation easily doesn't mean that
it has set up to do exactly what you wanted!

I tried to read many books, but the best book I have come across that
really helped me is book by F. Albert Cotton titled Chemical Applications of
Group Theory. This is very well written.

Now coming to your specific question: (Responses from Grant Hill and
Gerald Knizia are good starting point along with link provided)

Lets try to assign irreducible representation groups for  2s, 2px, 2py, and
2pz orbitals on oxygen atom of water molecule:

0) Orient water molecule so that z axis is 2 fold rotation axis and x axis
is normal to the plane of molecule and O atom is at origin

1) determine symmetry point group for water molecule: C2v (if you input the
correct structure, molpro or gaussian will detect it! or once you become
familiar with point groups you can detect it too!)

2) Once you know that point group, pull out character table for C2v point
group: Given in many text book or just google it

3) Now that you have character table: Let understand it first.
C2v    E       C2  sigma v (XZ)   sigma'v (YZ)
A1   1         1         1                1
A2   1         1        -1               -1
B1   1        -1         1               -1
B2   1        -1        -1                1

In the first column A1 A2 B1 and B2 are irreps of C2v point group. E, C2 (2
fold rotation), sigma v (XZ mirror plane) and sigma' v (YZ mirror plane) are
symmetry elements/operators.

4) Lets see the effect of E, C2, sig v and sig'v on 2S orbital (Try to
convince yourself that each of these are true)

E  on 2S: identity operator doesn't change any thing  =>  1
C2 on 2S: two fold rotation doesn't change any thing =>  1
sig v on 2S: XZ mirror plane """"                              => 1
sig' v on 2S: YZ mirror plance """""                           => 1

So 2S oxygen belongs to A1 irrep. (go to molpro manual and see what number
molpro assigns to this irrep)

Now lets consider 2Px orbital on O

E on 2Px : identity operator no change  => 1
C2 on 2Px: two fold rotation will change phase => -1
sig v on 2Px:  XZ mirror will not change anything => 1
sig'v on 2Px:  YZ mirror plane will change phase => -1

Now from the character table we can see that 2Px belongs to B1 irrep!

Lets work out one more 2Py on O
E (2Py) => 1 (this is trivial, right?)
C2(2Py) => -1 (will change phase)
sig v(2Py)  =>  -1 (will change phase)
sig'v(2Py)  => 1 (will not change anything)

So 2Py belongs to B2 irrep.

similarly you can work out 2Pz and orbitals on H (they are little bit more
involved as they will be linear combination of irreps)

So this is how you determine which orbital belongs to which irrep. Then you
can decide whether this orbital is occupied, empty or open etc..

I hope this helps you.

On Tue, May 31, 2011 at 2:27 AM, Wenjun Li <wli12 at ncsu.edu> wrote:

> Dear Molpro Users,
>
> I am a very new trial user of Molpro. I found that one key difference
> between Molpro and Gaussian is that, Molpro tries to solve the Schrodinger
> Equation as accurately as possible using all kinds of great methods to
> consider the electron correlation very well, especially using
> Multi-Reference Methods and lots of great and new methods developed from the
> authors of Molpro, this is great.
>
> However, to use Molpro is also becoming much more complicated than
> Gaussian, mainly because of two key reasons: (1) Molpro does not have a very
> great interface program for the input and output files editing and showing
> so far, certainly Molpro-View is great for this, but it is still not good
> enough at all comparing with GaussView, GaussView can almost let user to set
> up a calculation very quickly and easily, even though the user does not have
> a very good background on quantum chemistry. I heard of that, now Molpro
> developers are working on this kinds of interface program for Molpro, which
> will be definitely valuable for users. (2) Molpro requires the users to
> input lots of very details for solving the electronic structure calculation,
> including picking up different methods, options and directives, which really
> requires the users to have a very solid background on quantum chemistry, or
> else you will not really be able to use Molpro accurately and efficiently,
> just like me.
>
> I am one of the users who's background is not in quantum chemistry, I know
> very little on molecular symmetry stuff,  and I have only a very weak
> quantum chemistry background, which makes me lots of troubles to use Molpro.
> One very key difficulty for me so far is that, I can never figure out how to
> define *occ, closed and core n1,n2,n3,n4,n5,n6,n7,n8.* For example for *
> occ,n1,n2,n3,n4,n5,n6,n7,n8*, ni is the number of occupied orbitals in the
> irreducible representation i. This is the only very short description for
> OCC in Molpro Manuals, but I can never figure out how to define *occ,
> closed and core n1,n2,n3,n4,n5,n6,n7,n8.*
>
> So far what I understood is like that, *n1 is the # of sigma orbitals, **n2
> is the # of sigma* orbitals, **n3 is the # of pi-x orbitals, **n4 is the #
> of pi-x* orbitals, **n5 is the # of pi-y orbitals, **n6 is the # of pi-y*
> orbitals,* *then what is n7 and n8?* Am I right? Most likely I am wrong, I
> am actually always confused about how to Define the number of occupied
> orbitals in each symmetry. I believe this is most likely due to my weak
> quantum chemistry background. So may I ask for some suggestions from the
> Molpro-users. How can I define *occ, closed and core
> n1,n2,n3,n4,n5,n6,n7,n8.*? Where can I find some more references or
> descriptions for this? Or maybe can some one suggest me to read some kinds
> of textbook, so that I can understand this background info.
>
> Thanks a lot for all the help and suggestions in advance. Sorry for the
> long email, but maybe I did say something for very new users, who does not
> have a solid quantum chemistry background. Thanks a lot again.
>
>
> Best regards,
>
>
> Wenjun
> --
> *Wenjun LI *
> ---------------------------------------------------------------
> Chemical & Biomolecular Engineering,
> North Carolina State University,
> Engineering Building I, Box 7905,
> 911 Partners Way, Raleigh, NC 27695
> ---------------------------------------------------------------
>
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>
>

--
Regards,
Neeraj.
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