# Conversion of multiple DCD files into trr file for gmx rotacf calculation

**URL:** https://gromacs.bioexcel.eu/t/conversion-of-multiple-dcd-files-into-trr-file-for-gmx-rotacf-calculation/2765
**Category:** User discussions
**Created:** [September 4, 2021, 1:36pm UTC](https://gromacs.bioexcel.eu/t/conversion-of-multiple-dcd-files-into-trr-file-for-gmx-rotacf-calculation/2765 "2021-09-04T13:36:57Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![Santanu](https://avatars.discourse-cdn.com/v4/letter/s/6a8cbe/32.png) [@Santanu](https://gromacs.bioexcel.eu/u/Santanu)
#### Post date: [September 4, 2021, 1:36pm UTC](https://gromacs.bioexcel.eu/t/conversion-of-multiple-dcd-files-into-trr-file-for-gmx-rotacf-calculation/2765/1 "2021-09-04T13:36:57Z")

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GROMACS version: 5.1.5  
GROMACS modification: No  
Hi all,  
I am trying to calculate rotational correlation function of a protein, by using long trajectory length of ~ 100 ns (100 files each of 1 ns) DCD files genarted through NAMD followed by converting them into a trr file. For that, I am using catDCD and it is successfully generating trr file.  
But when I am using this trr file for gmx rotacf calculation, it is showing erros as:::  
**Failed to read magic number of trr file, this is not a trr file**

If anyone has any suggestion on how to get successful results on converted data (from DCD to trr), it is highly appreciated.

N.B.:: For a single DCD file, by loading it to vmd and thereby saving as trr is working fine (generating successful results for gmx rotacf), but saving 100 ns long trajectory in a single DCD file will be expensive in terms of memory.

SO , how to convert multiple dcd files into a gromacs readable trr file?

Thanks and regards  
Santanu