### Basic input file

Comment line O H 1 OH H 1 OH 2 HOH OH = 0.97 HOH = 118.00 *CFOUR(BASIS=AUG-PVTZ,SPHERICAL=ON,CALCLEVEL=MP2 MEMORY_SIZE=450000000)

The first line is a comment line. The second section is the molecular input. Notice there can only be one space. Additional spaces in the section will cause an error. The next section is the CFOUR keyword section.

### Optimization

Comment line O H 1 OH* H 1 OH* 2 HOH* OH = 0.97 HOH = 118.00 *CFOUR(BASIS=AUG-PVTZ,SPHERICAL=ON,CALCLEVEL=MP2 GEO_CONV=5,MEMORY_SIZE=450000000)

Here we have put * by the parameters we want optimized. The addition of GEO_CONV=*N* is used to define the convergence criteria, value is specified in Hartree/Bohr and the default is N=5 leading to a threshold value of 10**-5.

Additional options for optimizations can be found in the CFOUR manual. Additionally, you can use the force constants from a frequency calculation for the optimization. The FCMINT file from a frequency calculation just needs to be in the same directory as the optimization. CFOUR will automatically extract the force constants from the file.

### Transition State Optimization

transition-state search at HF/dzp level for the isomerization of HCN C N 1 R1* H 1 R2* 2 A* R1=1.15 R2=1.09 A=90. *ACES2(CALC=SCF,METHOD=TS,BASIS=dzp SCF_CONV=10,CONV=8)

### Frequency

Calculation of analytic second derivatives at RHF-SCF level O O 1 R1 H 1 R2 2 A H 2 R2 1 A 3 D R1 = 1.3925201514 R2 = 0.9482232889 A = 102.3524729106 D = 115.0411275427 *CFOUR(CALC=HF,BASIS=PVDZ,MEMORY=40000000

Since analytic second derivatives may not be available for every quantum mechanical method, one can estimate the second derivatives by finite difference which are controlled by the keywords FD_CALCTYPE, FD_STEPSIZE, FD_PROJECT, and FD_IRREPS.

### NMR Shifts

GIAO-MBPT(2)/GIAO-MP2 NMR shift calculation C F 1 RCF H 1 RCH 2 A H 1 RCH 2 A 3 D120 H 1 RCH 2 A 4 D120 RCF=1.3904961610 RCH=1.0834518879 A=108.6723004007 D120=120. *CFOUR(CALC=MP2,BASIS=dzp,PROP=NMR MEMORY=10000000 TREAT_PERTURBATION=SEQUENTIAL SCF_CONV=10)

### Reference

CFOUR, a quantum chemical program package written by J.F. Stanton, J. Gauss, M.E. Harding, P.G. Szalay with contributions from A.A. Auer, R.J. Bartlett, U. Benedikt, C. Berger, D.E. Bernholdt, Y.J. Bomble, L. Cheng, O. Christiansen, M. Heckert, O. Heun, C. Huber, T.-C. Jagau, D. Jonsson, J. Jusélius, K. Klein, W.J. Lauderdale, D.A. Matthews, T. Metzroth, D.P. O'Neill, D.R. Price, E. Prochnow, K. Ruud, F. Schiffmann, W. Schwalbach, S. Stopkowicz, A. Tajti, J. Vázquez, F. Wang, J.D. Watts and the integral packages MOLECULE (J. Almlöf and P.R. Taylor), PROPS (P.R. Taylor), ABACUS (T. Helgaker, H.J. Aa. Jensen, P. Jørgensen, and J. Olsen), and ECP routines by A. V. Mitin and C. van Wüllen.

### More information

For more detail, many example inputs and outputs, and to download the CFOUR manual visit the CFOUR website.