CRYSCOR A program for electron correlation in crystals
 
 

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Introduction

Introduction

 

Migen Halo, 2010-10-13

 

1. Basics

Briefly summarizing the main concepts: Since: we propose a perturbative (MP2) post-Hartree-Fock local correlation approach for (non-conducting) crystals, as currently implemented in the CRYSCOR program.
post-HF
the MP2 (Møller-Plesset at the second-order) perturbative correction currently implemented uses the HF solution as an uperturbed reference state; this solution is provided by the CRYSTAL program

local
local functions (called "Wannier functions, WF'') are adopted instead of delocalized Bloch functions; this permits the exploitation of the short-range nature (ER-6) of electron correlation. The main advantage of the local approach is the achievement of a scaling which is much lower (nearly linear - N or N2) than that of "classical'' MPn methods, whose scaling is typically ≥ N5. The scaling is the quantity of computational resources, in terms of time and memory, as a function of the irreducible number of atoms N of the system.

non-conducting
the localization procedure and the MP2 method itself are not suitable for conducting systems;

crystals
periodic structures have peculiar features, such as translational symmetry, thus their treatment is rather different from that of molecules.
Since CRYSCOR represents in a sense the post-HF option of CRYSTAL, a basic knowledge of this latter is required.
As far as concerns the theoretical background, refer to the publications and to the supporting material to be found in the CRYSCOR webpage; the more significant among them are listed here below:

 

2. Running CRYSCOR

As just said, CRYSCOR can be considered the post-HF option of the CRYSTAL program, since all the information at the HF level is provided by CRYSTAL. Therefore, to execute a CRYSCOR job, you first have to run a CRYSTAL calculation.

Actually, the CRYSTAL calculations to be run are two:

For the use of CRYSTAL please refer to the related Tutorials on the CRYSTAL website; here only the script usage is reported along with the CRYSCOR one.
The crystal and properties executables are to be run according to the following syntax (input file names to be used without extension):

runcry09 crystal_input_file

runprop09 crystal_input_file crystal_f9_unit

The script for running CRYSCOR calculations is runcryscor09; it needs 3 arguments: So:
runcryscor09 cryscor_input_file crystal_f9_unit properties_f80_unit

The extensions of the input and the output files needed/produced are summarized in table 1.

 

Table 1: Input and output standard file names.

executableinput fileoutput file
crystal.d12.out
  .f9
properties.d3.outp
  .f80
cryscor.d4.outc

 

4. General input comments

a. Basic input

The CRYSCOR input consists of a series of keywords (KW) followed by the respective arguments, to be written in free format. The order of KWs is pratically free.
A minimal CRYSCOR input file looks as follows (table 2):

 

Table 2: CRYSCOR input sample.

  
KNETCRYSTAL eigenvalue calculation
12 
MEMORYMemory required (megabytes)
2000 
DOMDEF Definition of the virtual space
1 
1 3 
1 2 3 
PAIRDefinition of the occupied space
6. 10. 
TOBJ Tolerances
0.001 0.001 
END 
END 
  

 

NEWK and MEMORY are the only mandatory cards to run a CRYSCOR calculation.

Recalling that:

in the input file:

PAIR, DOMAIN and TOBJ are then the fundamental parameters of a CRYSCOR calculation. The complete list of cards is reported in the CRYSCOR Manual.

b. When approximations are activated...

The computation of bielectronic integrals represents the most expensive part of the calculation, that is why in CRYSCOR are implemented two very efficient approximated techniques for their estimation, namely Density Fitting (DF) and the Multipolar (MP) technique. The two approximations are complementary, since DF is particularly suited for close-by WF-WF pairs, whereas MP can be safely used only for large distances.

 

4. Exercises proposed

In the following exercises are proposed on different types of systems, i.e. LiH as a representative of ionic crystals, H2O polymer to check the relation molecular-periodic, NH3 as a representative of molecular crystals, and Argon adsorbed on MgO to show the treatment of adsorption phenomena. The different tutorial sections can be chosen from the menu on the left at the top of this page.

LiH and NH3 tutorial sessions are specially designed to teach the user to prepare the input and read the CRYSCOR output.
Furthermore, in NH3 are proposed:

 
Last Modified: Monday, February 10, 2014 [16:01:26].