On Sat, Sep 25, 2010 at 5:01 AM, <= david.anick{}rcn.com> wrote:

Dear Reynier,

What is hap= pening is that when you have a ring, you can't just make the
dihedral= angles anything you want, because there are mathematical relations amon= g them. To see this, consider a special case where all the atoms l= ie in a plane. All five dihedral angles equal 0. If you cons= train all five angles to be zero, Gaussian is happy to optimize this.&nb= sp; Now suppose you change one of the angles, even if you only change it= to +1 degree. It becomes geometrically IMPOSSIBLE. Because = four of the five dihedrals are still zero, all five atoms are still forc= ed to lie in a single plane. Then a mathematical consequence is th= at the fifth dihedral is automatically zero also. If you tell it t= o make that dihedral equal to +1, you are asking Gaussian to find a geom= etry that mathematically cannot exist. Guess what: Gaussian cannot= do it, and gives you an error message. The error message is appro= priate: "error imposing constraints".

If you want to change o= ne dihedral angle a little, you must allow some of the other four dihedr= al angles to adapt to the change. If you work a little with this, = you will see that two dihedral angles essentially fix a five-member ring= . (This is technically true only if the bond lengths and bond angles are= also fixed, but it is hard to adapt bond lengths and angles to accommod= ate changing dihedral angles.) Try this: change the dihedral angle= you want to change, and remove three of the other constraints, so that = you are specifying only two of the five dihedral angles. I think G= aussian will be happy with that and will be able to converge. If t= hat works maybe you can try specifying three of the dihedrals, or concei= vably, four, if the changes are very small.

You need to think= about what is the question you are trying to answer. If it's abou= t the flexibility of the ring, your best approach may be to constrain ju= st one dihedral, and let the rest of the ring adapt as it needs to.
<= br>
I hope this has been helpful.
Peace,
David Anick PhD MD
=
---- Original message ----

Date: Fri, 24 Sep 2010 21:27:14 +0200<= br>From: "Reynier Suardiaz del R=EDo reynier.suardiaz!^!gmail.com" <owner-chemistry{}ccl.n= et>

Subject: CCL:G: freezen dihedrals in five-membe= red rings

<= b>To: "Anick, David " <david.anick{}rcn.com>

Dear D. = Close

Many thanks for your answer, you was right. I typed the= values of the dihedrals with 6 decimal places exactly matching with tho= se of the input structure. Doing this, the geometry optimization have fi= nished without problem in a few iterations. Now what I would like to do = is the following: I want to generate diferent conformations of this fura= nose ring by changing the dihedrals (between permitted values without br= eaking of the ring) and partially optimize this structures (obtained by = slightly changing the dihedral values) and keeping frozen the five dihed= rals. When I try to do this using redundant coordinates in gaussian I ob= tained the same error message than before:

-------------
&= nbsp;Iteration 99 RMS(Cart)=3D 0.00005822 RMS(Int)=3D 0.0095= 5580
Iteration100 RMS(Cart)=3D 0.00005748 RMS(Int)=3D&nbs= p; 0.00959825
New curvilinear step not converged.
Erro= r imposing constraints
Error termination via Lnk1e in C:\G03W\l= 101.exe at Fri Sep 24 21:05:06 2010.
Job cpu time: 0 days= 0 hours 0 minutes 1.0 seconds.
File lengths = (MBytes): RWF=3D 7 Int=3D = ; 0 D2E=3D 0 Chk=3D = ; 8 Scr=3D 1
--= ------------

even if I only change one dihedral from its original= value (at the input geometry) in less than one degree, the calcul= ation ends with the above error message.
Does anybody knows how to do= this in gaussian, I mean, changing the dihedral angles of a five memebe= red ring (from its text input file) and to performe a partial geometrica= l optimization with diferent dihedral angles, other than the one of the = input geometry?

any comment or suggestions are welcome.

th= anks in advance and with very best regards

Reynier

2010/9/22 Close, David M. CLOSED~!~mail.etsu.edu <owner-chemistry]*[ccl.net>

Sent t= o CCL by: "Close, David M." [CLOSED#,#mail.etsu.edu]
Reynier:
There is no li= mit to how many dihedrals you can freeze. The problem is that you = typed something wrong. Notice that the program tried 99 iterations= to fit you frozen coordiate information into the optimization routine.<= br>

Either you connected the coordinates incorrectly, or did = not have enough precision in the frozen coordinate.
So if the input l= ine has something like 10 5 6 8 31.3, first look at the string 10= 5 6 8 and make sure this is correct. The use a graphics program t= o examine the actual dihedral geometry. Run through the 4 atoms in= the string 10 5 6 8 and see what the graphic program thinks the dihedra= l angle actually is. Copy the value to 5-6 decimal places and re e= nter the data.

If this doesn't work, then you have= to use trial and error. You said that freezing 2 dihedrals works.= But how many iterations did it take? I would expect only 2-= 3. If more, then refine the coordinates, and then add a third froz= en dihedral. You can quickly find the offending entry when the opt= imization routine bombs.

Regards, Dave Close.
<= br>________________________________________
> From: owner-chemistr= y+closed=3D=3Detsu.edu= [A]ccl.net [owner-chemi= stry+closed=3D=3Detsu.edu<= /a>[A]ccl.net] on behal= f of Reynier Suard az reynier.suardiaz(a)gmail.com [owner-chemistry[A]ccl.net]

Sent: Wednesday, Septe= mber 22, 2010 10:49 AM
To: Close, David M.
Subject: CCL:G: freezen= dihedrals in five-membered rings

Sent to CCL by: "Reynier &= nbsp;Suard az" [reynier.suardiaz]_[gmail.com]
Dear All

I want to generate a l= ot of conformations of furanose ring (or cyclopentane?) and later partia= lly optimize them but keeping frozen the dihedral angles. I am trying to= use redundant coordinates in gaussian writing at the end of the input g= aussian file the desired dihedrals to keep frozen. I am receiving an err= or message when i try to keep frozen more than two dihedral angles (of t= he ring) at the same time. For example if I try to froze the five dihedr= als of the ring I get the following message:

------ Iteration 99 RMS(Cart)=3D 0.00001156 RMS(Int)=3D 0.0= 0309967
Iteration100 RMS(Cart)=3D 0.00001134 RMS(Int)=3D = 0.00310385
New curvilinear step not converged.
E= rror imposing constraints
Error termination via Lnk1e in C:\G03= W\l101.exe at Mon Sep 20 17:48:17 2010.
Job cpu time: 0 d= ays 0 hours 0 minutes 1.0 seconds.
File lengt= hs (MBytes): RWF=3D 7 Int=3D &nb= sp;0 D2E=3D 0 Chk=3D 7 Scr=3D &n= bsp; 1
---------

I receive this error message eve= n when I try to freeze the dihedral at the same value they already have = in the initial structure.

Is not possible what am I trying to do?= How can I overcome this problem with gaussian? Is there any other possi= bility to do this kind of partial optimization in five-membered rings? N= ote that I can not freeze all the dihedrals using optimization in intern= al coordinates (opt=3Dz-matrix with a separate input section of "constan= ts") because of to define a z-matrix of a five-membered ring are necesar= y only two dihedrals, so I have to use redundants.

any co= mments or sugestions would be appreciatte

thanks in advance and v= ery best regards

Reynierhttp://www.ccl.net/cgi-bin= /ccl/send_ccl_messagehttp://www.ccl.net/chemistry/sub_unsub.shtmlhttp://= www.ccl.net/spammers.txt

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