:orphan:
CAM functional
==============
In DIRAC, you can calculate with the CAMB3LYP functional using::
.DFT
CAMB3LYP
It is also possible to change the default parameters. The following line
will reproduce the above functional::
.DFT
CAM p:alpha=0.19 p:beta=0.46 p:mu=0.33 x:slater=1 x:becke=1 c:lyp=0.81 c:vwn5=0.19
but it will give you complete freedom over the parameters. As always, please
verify your results carefully when changing these parameters.
Approaching the B3LYP limit
---------------------------
CAM uses the following partitioning of the two-electron interaction:
.. math::
\frac{1}{r_{12}} = \frac{1 - [\alpha + \beta erf (\mu r_{12})}{r_{12}}
+ \frac{ [\alpha + \beta erf (\mu r_{12})}{r_{12}}
For testing purposes we can try to approach the B3LYP limit
using the CAM code, in order to check that the alpha/beta limits work well.
In B3LYP, "HF" admixture is 0.2 so in CAM this can be obtained with
alpha=0.2 and beta=0.0.
So this is B3LYP::
.DFT
GGAKEY Slater=0.8 Becke=0.72 HF=0.2 LYP=0.81 VWN=0.19
Now naively we could try to obtain B3LYP results like this (**this is wrong**)::
.DFT
CAM p:alpha=0.2 p:beta=0.0 p:mu=0.0 x:slater=0.8 x:becke=0.72 c:lyp=0.81 c:vwn5=0.19
**This does not work in DIRAC** and the correct B3LYP expressed using CAM in DIRAC can be obtained
like this::
.DFT
CAM p:alpha=0.2 p:beta=0.0 p:mu=0.0 x:slater=1.0 x:becke=0.9 c:lyp=0.81 c:vwn5=0.19
This is because x:slater and x:becke get scaled inside fun-cam.c by (1-alpha).
The x:becke=0.9 can be surprising if you read the paper by Yanai et al. [Chem.
Phys. Lett. 393 (2004) 51] and follow their examples.