Geometry Optimisation
As far as I know there is no systematic study of the geometry optimisation of small molecules using a plane-wave psuedopotential code. Probably because it would be an incredibly dull and tedious task. However I needed the geometry of a range of small molecules I thought I'd collect my results.
warningBefore comparing these results with other methods some factors must be taken into account.
- No symmetry constriants were used. This is to make the results more applicable to large systems which often lack symmetry. Consequently the values for say all four C-H bonds in CH4 are given
- The results are just to give an indication of the level of accuracy that can be expected from such codes. Slightly differant results will be obtained for different psuedo-potentials / exchange correlation functional / initial starting configurations. Don't trust my results, generate your own!
Bond lengths in Angstrams
Bond angles in Degrees
Dipole Moments in debyes (1D=3.336E-30 Cm)
Results with Ultra-soft pseudopotentials (MSI database) and PW91 functional
Methane CH4
| Castep | Expt |
| C-H | 1.0879 | 1.0870 |
| C-H | 1.0882 | |
| C-H | 1.0874 | |
| C-H | 1.0880 | |
| CCH | 109.36 | 109.47 |
| CCH | 109.42 | |
| CCH | 109.58 | |
| CCH | 109.51 | |
| dipole | 2.490E-3 | 0.0 |
Convergance info and CASTEP output file are avalable. Also the geometry in xyz format
Ethane C2H6
| Castep | Expt |
| C-C | 1.5120 | 1.5351 |
| C-H | 1.0912 | 1.0940 |
| C-H | 1.0913 | |
| C-H | 1.0913 | |
| C-H | 1.0914 | |
| C-H | 1.0911 | |
| C-H | 1.0908 | |
| CCH | 111.72 | 111.7 |
| CCH | 111.66 | |
| CCH | 111.51 | |
| CCH | 111.32 | |
| CCH | 111.59 | |
| CCH | 111.41 | |
| HCCH | 180.04 | 180.0 |
| HCCH | 179.92 | |
| HCCH | 179.85 | |
| dipole | 1.4038E-3 | 0.0 |
Convergance info and CASTEP output file are avalable. Also the geometry in xyz format
Ethene C2H4
| Castep | Expt |
| C-C | 1.3172 | 1.339 |
| C-H | 1.0829 | 1.087 |
| C-H | 1.0833 | |
| C-H | 1.0832 | |
| C-H | 1.0836 | |
| CCH | 121.94 | 121.3 |
| CCH | 121.76 | |
| CCH | 121.88 | |
| CCH | 121.70 | |
| HCH | 116.41 | |
| HCH | 116.29 | |
| dipole | 2.577E-3 | 0.0 |
Convergance info and CASTEP output file are avalable. Also the geometry in xyz format
Ethyne C2H2
| Castep | Expt |
| C-C | 1.1947 | 1.203 |
| C-H | 1.0618 | 1.060 |
| C-H | 1.0622 | |
| CCH | 179.96 | 180 |
| CCH | 179.87 | |
| dipole | 4.78999E-3 | 0.0 |
Convergance info and CASTEP output file are avalable. Also the geometry in xyz format
Carbon Monoxide CO
| Castep | Expt |
| C-O | 1.1435 | |
| dipole | 0.1470318 | 0.117 |
Convergance info and CASTEP output file are avalable. Also the geometry in xyz format
Ozone O3
| Castep | Expt |
| O-O | 1.2778 | 1.2716 |
| O-O | 1.2778 | |
| OOO | 118.30 | 117.47 |
| Dipole | 0.6032 | 0.534 |
Convergance info and CASTEP output file are avalable. Also the geometry in xyz format
Allene CH2=C=CH2
| Castep | Expt |
| C=C | 1.2922 | 1.3084 |
| C=C | 1.2918 | |
| C-H | 1.0842 | 1.087 |
| C-H | 1.0837 | |
| C-H | 1.0844 | |
| C-H | 1.0841 | |
| CCC | 179.59 | 180 |
| CCH | 121.61 | |
| CCH | 121.34 | |
| CCH | 121.38 | |
| CCH | 121.53 | |
| HCH | 117.05 | 118.2 |
| HCH | 117.09 | |
| HCCH | 90.49 | 90 |
| HCCH | 89.68 | |
| Dipole | 1.6705E-2 | 0 |
Convergance info and CASTEP output file are avalable. Also the geometry in xyz format
Acetonitrile CH3CN
| Castep | Expt |
| C-N | 1.4326 | 1.468 |
| C-C | 1.1697 | 1.159 |
| C-H | 1.0899 | 1.107 |
| C-H | 1.0909 | |
| C-H | 1.0906 | |
| CCN | 179.68 | 180 |
| CCH | 110.70 | 109.7 |
| CCH | 110.54 | |
| CCH | 110.64 | |
| HCH | 108.32 | |
| HCH | 108.41 | |
| HCH | 108.15 | |
| Dipole | 4.1486303 | 3.924 |
Convergance info and CASTEP output file are avalable. Also the geometry in xyz format
Methylamine CH3NH3
| Castep | Expt |
| C-N | 1.4496 | 1.471 |
| C-H | 1.0994 | 1.099 |
| C-H | 1.0911 | |
| C-H | 1.0914 | |
| N-H | 1.0214 | 1.010 |
| N-H | 1.0215 | |
| HNH | 107.87 | 107.1 |
| HCN | 115.50 | |
| HCN | 109.54 | 108.0 |
| HCN | 109.31 | |
| CNH | 112.27 | 110.3 |
| CNH | 112.33 | |
| dipole | 1.2302 | 1.31 |
Convergance info and CASTEP output file are avalable. Also the geometry in xyz format