### Author Topic: Effective Core Potentials + RIRPA  (Read 11250 times)

#### jeb

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##### Effective Core Potentials + RIRPA
« on: April 20, 2013, 07:37:55 PM »
Hello,

Thank you for your interest in using RIRPA. During the course of the development, however, Effective Core Potentials (ECPs) were overlooked in the initial implementation and were not enabled in the Hartree-Fock Energy portion of the code. This has been corrected, and will be available in the next release, but does not help those who are already trying to use the present version.  What follows here is a set of instructions for computing the RIRPA energy when using ECPs.  The attachment is a prepared control file for H2, and a text file containing the different energy contributions to the RIRPA total energy. As a simple test, please use the h2control file (be sure to rename it control after download) and follow the steps below to compute each piece of the energy.  Since this test uses the RI-J approximation, you will need to use ridft for steps 1 and 3. Compare your results to the ones in the h2energies.txt file, and if they agree then you have properly performed an RIRPA calculation.  While the test does not contain ECPs, the procedure is identical even for systems that require ECPs.
I hope this enables you to carry out your calculations successfully. If you have further questions or comments about the use of ECPs and RIRPA, please reply to this post.

Best Regards,
Jefferson E. Bates
Developer
UC Irvine, California, USA

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#########     Steps for Computing the RIRPA Energy     ##########
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1) Perform a semi-local DFT calculation using a non-hybrid functional on your reference structure to obtain a converged set of Kohn-Sham molecular orbitals.
Typically PBE and TPSS are the optimal choices for this step.  While the differences in the RPA energy are generally small, Eshuis et al.(http://link.springer.com/article/10.1007%2Fs00214-011-1084-8) demonstrated a slight dependence of barrier heights on the choice of semi-local functional, so be sure to report which functional you have used to generate your reference.  The RI-J approximation may be used in this step to accelerate the calculation.

2) Perform an RIRPA correlation energy calculation using the RIRPA module.  To compute just the correlation energy, you must add the following to the control file:
\$rirpa
npoints 30
nohxx
The number of grid points, 30 in this example, can be changed depending on the details of your system and the desired sensitivity measure in the correlation energy.  Larger grids are necessary to study small gap systems, or when the core is not kept frozen.  The additional nohxx option disables the computation of the Hartree-Fock energy (HXX energy in the output of the program) so that only the RPA correlation energy will be computed.  You must specify a cbas and a jbas in the control file for this step to work properly.

3) Perform a single iteration of a Hartree-Fock calculation to extract the HXX energy from the semi-local reference orbitals.
WARNING: This step will overwrite the converged DFT orbitals obtained in step 1 if you are not careful.    In order to avoid this, either copy
the contents of your working directory to a new directory where you can perform this step, or copy the mos or alpha/beta files to a safe place where you can restore them after you have computed the HXX energy.  To actually compute the energy, change the number of scf iteractions to 1, either by directly modifying \$scfiterlimit in the control file or by using the "scf" menu in define. Next, turn off the DFT flag either by removing (or commenting out) the \$dft and func/grid lines from the control file or by using define and choosing "off" in the "dft" menu at the end of define. After these preparations, you are ready to compute the HXX energy.  If you have enabled the RI-J approximation, be sure to use ridft, otherwise use dscf.  The total energy reported in the output of this calculation is the HXX energy needed for the RIRPA total energy.  If you have performed this calculation in a new directory, you can move the necessary output file back to the original directory and delete the directory used to compute the HXX energy.  If you have chosen to copy your mos or alpha/beta files to a safe place, be sure to restore them AND the control file to its original state to avoid confusion.

4) After you have computed the RIRPA correlation energy in step 2), and the HXX energy in step 3), compute the total RIRPA energy by summing these two numbers together.

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