# Coulomb (SR) calculated by manual and Gromacs are inconsistent

**URL:** <https://gromacs.bioexcel.eu/t/coulomb-sr-calculated-by-manual-and-gromacs-are-inconsistent/2609>\
**Category:** User discussions\
**Created:** [July 29, 2021, 6:31am UTC](https://gromacs.bioexcel.eu/t/coulomb-sr-calculated-by-manual-and-gromacs-are-inconsistent/2609 "2021-07-29T06:31:17Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![Ymw](https://avatars.discourse-cdn.com/v4/letter/y/5daacb/32.png) [@Ymw](https://gromacs.bioexcel.eu/u/Ymw)\
**Post date:** [July 29, 2021, 6:31am UTC](https://gromacs.bioexcel.eu/t/coulomb-sr-calculated-by-manual-and-gromacs-are-inconsistent/2609/1 "2021-07-29T06:31:17Z")

</div>

GROMACS version:gmx2020.3  
GROMACS modification: Yes

In order to figure out the calculation of Coulomb in gromacs, I manually calculated the Coulomb-14 and Coulomb (SR) values of an ethanol molecule and compared them with the calculation results of gromacs.  
In gromacs calculations, the box size is about 4nm. coulombtype = cut-off, rcoulomb = 2. The manual calculation method of Coulomb-14 is f=0.5, if i, j is 1,4. The calculation method of Coulomb (SR) is f=0, if i, j is 1,2 or 1,3; f= 0.5, if i, j is 1,4; otherwise f=1.

The manual calculation result of Coulomb-14 is consistent with gromacs, but the calculated value of Coulomb (SR) is inconsistent.

**Gromacs calculation**  
Coulomb-14=-30.969759，Coulomb (SR)= 36.379906.

**Manual calculation**  
Coulomb-14=-30.973276，Coulomb (SR)= 5.3962189

May I ask what causes this problem?

---

<div class="post-metadata">

**Author:** ![Ymw](https://avatars.discourse-cdn.com/v4/letter/y/5daacb/32.png) [@Ymw](https://gromacs.bioexcel.eu/u/Ymw)\
**Post date:** [July 29, 2021, 8:16am UTC](https://gromacs.bioexcel.eu/t/coulomb-sr-calculated-by-manual-and-gromacs-are-inconsistent/2609/2 "2021-07-29T08:16:30Z")

</div>

solved. Coulomb (SR) does not include Coulomb-14, so the energies of i, j=1, 4 should be removed when calculating by hand.

---

<div class="post-metadata">

**Author:** ![alevilla](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/alevilla/32/439_2.png) [@alevilla](https://gromacs.bioexcel.eu/u/alevilla)\
**Post date:** [August 20, 2021, 8:05am UTC](https://gromacs.bioexcel.eu/t/coulomb-sr-calculated-by-manual-and-gromacs-are-inconsistent/2609/3 "2021-08-20T08:05:22Z")

</div>

Hi,  
In the “GROMACS calculation” Coulomb (SR) does not include Coulomb (1-4), while it does in your manual calculation.  
Best regards  
Alessandra