Wenli,<br><br> 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!<br>
<br> 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.<br><br> Now coming to your specific question: (Responses from Grant Hill and Gerald Knizia are good starting point along with link provided) <br>
<br>Lets try to assign irreducible representation groups for 2s, 2px, 2py, and 2pz orbitals on oxygen atom of water molecule: <br><br>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<br>
<br>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!)<br><br>2) Once you know that point group, pull out character table for C2v point group: Given in many text book or just google it<br>
<br>3) Now that you have character table: Let understand it first.<br>C2v E C2 sigma v (XZ) sigma'v (YZ)<br> A1 1 1 1 1<br> A2 1 1 -1 -1<br>
B1 1 -1 1 -1<br> B2 1 -1 -1 1 <br><br>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.<br>
<br>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)<br><br>E on 2S: identity operator doesn't change any thing => 1<br>C2 on 2S: two fold rotation doesn't change any thing => 1<br>
sig v on 2S: XZ mirror plane """" => 1<br>sig' v on 2S: YZ mirror plance """"" => 1<br><br>So 2S oxygen belongs to A1 irrep. (go to molpro manual and see what number molpro assigns to this irrep)<br>
<br>Now lets consider 2Px orbital on O <br><br>E on 2Px : identity operator no change => 1<br>C2 on 2Px: two fold rotation will change phase => -1<br>sig v on 2Px: XZ mirror will not change anything => 1<br>sig'v on 2Px: YZ mirror plane will change phase => -1<br>
<br>Now from the character table we can see that 2Px belongs to B1 irrep!<br><br>Lets work out one more 2Py on O<br>E (2Py) => 1 (this is trivial, right?)<br>C2(2Py) => -1 (will change phase)<br>sig v(2Py) => -1 (will change phase)<br>
<div class="gmail_quote">sig'v(2Py) => 1 (will not change anything)<br><br>So 2Py belongs to B2 irrep.<br><br>similarly you can work out 2Pz and orbitals on H (they are little bit more involved as they will be linear combination of irreps)<br>
<br>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..<br><br>I hope this helps you.<br><br><br><br><br>On Tue, May 31, 2011 at 2:27 AM, Wenjun Li <span dir="ltr"><<a href="mailto:wli12@ncsu.edu">wli12@ncsu.edu</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">Dear Molpro Users,<br><br>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. <br>
<br>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.<br>
<br>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 <b>occ, closed and core n1,n2,n3,n4,n5,n6,n7,n8.</b> For example for <b>occ,n1,n2,n3,n4,n5,n6,n7,n8</b>, 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 <b>occ, closed and core n1,n2,n3,n4,n5,n6,n7,n8.</b> <br><br>So far what I understood is like that, <b>n1 is the # of sigma orbitals, </b><b>n2 is the # of sigma* orbitals, </b><b>n3 is the # of pi-x orbitals, </b><b>n4 is the # of pi-x* orbitals, </b><b>n5 is the # of pi-y orbitals, </b><b>n6 is the # of pi-y* orbitals,</b> <b>then what is n7 and n8?</b> 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 <b>occ, closed and core n1,n2,n3,n4,n5,n6,n7,n8.</b>? 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.<br>
<br>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.<br><br>
<br>Best regards,<br><br><br>Wenjun<br><font color="#888888"><h3><a name="130451a091399631_SECTION000202100000000000000"></a></h3>-- <br><b>Wenjun LI </b><br>---------------------------------------------------------------<br>
Chemical & Biomolecular Engineering,<br>
North Carolina State University,<br>Engineering Building I, Box 7905,<br>911 Partners Way, Raleigh, NC 27695<br>---------------------------------------------------------------<br>
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<br></blockquote></div><br><br clear="all"><br>-- <br>Regards,<br>Neeraj.<br><br><br>