Hello!

You are right. Water is not linear and you should have 3N-6 normal modes. Perhaps you found values for a linear structure. Then you'd have 3N-5, hence four frequencies. Questios: Are all values real?, what is the structure they report? What is the reference? Gas phase or liquid water?

Best wishes,

JL

Afterthought:

You might have seen "combination" bands involving these modes. I do suggest you check some recent analysis of the experimental IR spectrum for H2O in the gas phase.

Thank you very much for help, Professor.

I'm just start to learn gaussian, and this is part of homework project.

Yes. one of the result is negative, so I think the extra one is imaginary.

force constants of water/ aJ/A2,aJ/(A rad),aJ/rad2

Fr 8.545

Frr -0.101

Fa 0.697

Fra 0.237

Reference: Transferable scaling factors for density functional derived vibrational force fields. J.Phys.Chem.1995,99,3093-3100.

Thanks. Have a good day.

These are force constants in internal coordinates. With two equivalent stretchings (r) and a bending (a) the 3x3 force constant Matrix has 4 different elementi (in place of 6)

Fr Frr. Fra

Frr Fr Fra

Fra Fra. Fa

Of course diagonal elements are positive but off-diagonal elements can be negative

Thank you very much, Professor.

Can I ask how to calculate the internal coordinates force constants?

When I used "opt freq" in Gaussian09, the results I got is

1 2 3

A1 A1 B2

Frequencies -- 1730.9149 3713.8555 3812.4340

Red. masses -- 1.0837 1.0442 1.0808

Frc consts -- 1.9131 8.4854 9.2557

I read the manual, and tried some other keywords, but don't really know how.

Using Freq=Internal gives you an output where the normal modes are printed in internal instead of Cartesian coordinates. You then need to find the transformation matrix between them and apply that to your force constant matrix. That's probably best done using Matlab or something equivalent, but it's not as trivial as just picking out the values from the output file. Is this really what you are supposed to do in your homework?

Thank you very much. It is a literature homework, small project.

I understand this better now. I got the internal force constant matrix. So the Gaussian cannot directly output internal force constants?

This is my first time studying Computational Chem.

Can I ask a more general question about the Gaussian09, the paper usually just state what they used, such as the software and method, basis sets, and the theories. How can I got the details when reproduce the data at computer?

Like after read the manual, I know I can write "scale=0.xxx"to scale the frequency. But I can't find instruction about SQM scale method.

Usually authors must give enough information in a publication so that the reader can reproduce their results. In straightforward tasks such as geometry optimizations and frequency calculations, it should probably be enough in most cases to mention the method and basis set and then show the structures that were computed; often, authors also supply the optimized coordinates as supplementary information. If they apply some scaling, it is mandatory to also list the scaling factors or supply some reference from where they can be gleaned. However, don't hesitate to write the authors of some publication if you are unable to reproduce their results or feel that their text is unclear about some details.

I admit it's the first time I heard about the SQM scaling method, but it seems that it is a bit different to the scaling method that Gaussian supports with the "scale" keyword. What Gaussian does is a very basic scheme which applies a common scaling factor to all harmonic frequencies after the frequency calculation has been carried out. If I undestand correctly, the SQM method seems to scale individual force constants differently for different types of internal coordinates. This could probably be done manually by extracting the force constant matrix from a Gaussian checkpoint file, transforming it to internal coordinates and applying the scaling, but it seems a bit laborious and I don't think it is something that the software supports by itself.

Thanks a lot for helping. I have learned a lot. :)

Xie,

there is a method used by Nicolai Lehnert in Michigan Ann Arbor chemistry where he uses a code to extract the force constants by defining UB matrix using the internal modes. You may want to look that up. He used to call it QCNCA at some point. I don't know what it is called now.

Given that the question was based on a project/homework, this thread is probably a little out of date, but I thought I would add that you can extract the Force Constant Hessian matrix from a formatted checkpoint file (fchk). First convert the .chk generated from the opt freq calculation using:

formchk file.chk outfile.fchk

Then put together a very small text file (I called mine FC.inp) containing the text:

outfile.fchk

Cartesian Force Constants

quit

Use this file on the command prompt as an input into the demofc script that comes with Gaussian 09:

demofc hessian.out &

Inside the hessian.out file will be the cartesian force constants. 

I am still quite new to Gaussian, so I'm not figuring out how to generate specific bond force constants for Amber force fields. Slowly getting there. 

Hope this helps someone. 

Hi Anthony,

My problem is similar. I trying to generate specific force constants of a particular bond for a force field. I have gotten the Cartesian Force constants, but I don't know how to proceed further. Could you possibly shed some light?

I am currently writing some software which will automate this from a Gaussian opt freq calculation. Once I have finished I will point you in its direction. 

i am using siesta code to create force constant matrix but i still have problem with big matrix, if any one have some explain can help me

Antony Nash, I have a very similar problem to solv. I need to generate specific force constants for bonds and angles for a MM calculation. Did you have some news about your software?

Cheers!

Mayank Boob

J. M. Seminario (1996) Calculation of intramolecular force fields from second-derivative tensors. Internat. J. Quant. Chem. 60:1271-1277

Hi all,

all you need in order to do a compliance or relaxed force constant calculation is a log file of the relevant g09 frequency calculation. The archive entry must have the cartesian force constants.

Than, install our Compliance code.

A) Pre-installation tasks (if needed)

1) sudo apt-get install g++

2) sudo apt-get install gtkmm-2.4-dev

3) sudo apt-get install gtkglextmm-x11-1.2-dev

4) sudo apt-get install gfortran

5) sudo apt-get install libblas-dev liblapack-dev

6) sudo apt-get isnatll libgmm++-dev

B) Installation

cd into the compliance-3.0.2 directory sudo

configure sudo

make sudo

make install

C) Running

1) run the executable: ./compliance-3.0.2/src/compliance

2) load any g09 freq.log file (change into full screen mode, in order to see the „file open“ menu)

Just right click on the atom-atom system you are interested.

Have fun!

Jörg