# Help file for the Mrgddb utility of the ABINIT package.

## This file explains the i/o parameters needed for the derivative database (DDB) merging code (MRGDDB).

The user is advised to be familiar with the main abinit help file before reading the present file. It is important to read also the Analysis of Derivative DataBase code (Anaddb) help file to complement the present reading.

##### Copyright (C) 1998-2017 ABINIT group (XG,DCA)

### Content of the help file:

- 1. Introduction
- 2. What is the usefulness of a merging code ?
- 3. How does the merging code work ?

### 1. Introduction

The mrgddb code has the purpose to merge "transfer" DDBs (that were generated from the ABINIT code) to make a complete DDB that can be exploited by the Anaddb code.

The input is very simple, and could be given directly at the screen, or more conveniently, piped from a file. The user should provide first the name of the new (output) DDB. He/she should then give a short description (one line) of this new DDB that will be created. This line will be printed at the beginning of the DDB.

The user should then give the number of DDBs that will be merged, then the whole set of filenames for the DDBs to be merged, one on each line.
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### 2. What is the usefulness of a merging code ?

The ABINIT code in its present version is only able to produce results for one q wavevector for each dataset. A database for more than one q point can thus be created using MRGDDB. Also, it is useful to be able to merge different DDBs if they are produced independently on different machines.
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### 3. How does the merging code work ?

The DDBs are made of two parts. The first part is a list of the parameters that were used to make the DDB, and the second part lists the 2DTEs and 3DTEs. The merging code will check if the following variables are exactly the same in the different input DDB : natom, ntypat, nband, acell, amu, ecut, ixc, lloc, ngfft, occ, rprim, typat, xred, zion. For nband and occ, the value of occopt is taken into account (see abinit help file). If possible, MRGDDB will produce a DDB with occopt=0. In case of two different data sets, the code will print an error message and stop. The code cannot merge two DDB that have been generated using two different geometries or convergence (ecut, ...) parameters. The only exception is connected to the possibility to use Time-reversal symmetry to decrease the number of special k points when the wavevector of the perturbation is Gamma. In that case, the code will merge the DDBs and put the largest set of k-points inside the new DDB. MRGDDB will copy the latest date of the transfer or current DDB and copy it in the new DDB. It will also take the less accurate tolwfr and copy it in the new DDB. This ends the first part of the action of MRGDDB, namely to compare the information of the two different DDB. When the checking is done, MRGDDB will check the content of the different data blocks and constitute the new DDB by copying sequentially the non-identical blocks and merging the identical blocks. In case two elements are identical, MRGDDB copies the value of the transfer DDB. (This latter property makes it easy to get rid of old, erroneous data and put new, correct data in its place) Finally, the summary of the block content of the DDB is provided at the end of the DDB file.
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