Dear Andris Gulans
Thank you for your help.
I increased lmaxvr, lmaxmat and lmaxapw to '10' and gmaxvr to 14 but fluctuations in RMS have remained and the scf was not converged:

RMS change in effective potential (target) : 0.159117E-05 ( 0.100000E-05)
RMS change in effective potential (target) : 0.131828E-05 ( 0.100000E-05)
RMS change in effective potential (target) : 0.131802E-05 ( 0.100000E-05)
RMS change in effective potential (target) : 0.117463E-05 ( 0.100000E-05)
RMS change in effective potential (target) : 0.122905E-05 ( 0.100000E-05)
RMS change in effective potential (target) : 0.123839E-05 ( 0.100000E-05)
RMS change in effective potential (target) : 0.202334E-05 ( 0.100000E-05)
RMS change in effective potential (target) : 0.142195E-05 ( 0.100000E-05)
RMS change in effective potential (target) : 0.149716E-05 ( 0.100000E-05)
RMS change in effective potential (target) : 0.183803E-05 ( 0.100000E-05)
RMS change in effective potential (target) : 0.174085E-05 ( 0.100000E-05)

Here I print input.xml for more detail.

<input>

<title>organic crystal</title>

<structure speciespath="/opt/exciting/species" autormt="false">

<crystal>
<basevect>16.096457122 0.000000000 0.000000000</basevect>
<basevect>0.000000000 11.331897047 0.000000000</basevect>
<basevect>-0.57538792 0.000000000 23.049303126</basevect>
</crystal>
<species speciesfile="C.xml" rmt="1.16">
<atom coord="0.32218239 0.11269320 0.94013915"/>
<atom coord="0.67781676 0.88730712 0.05986037"/>
<atom coord="0.17784018 0.61269894 0.55990759"/>
<atom coord="0.82216083 0.38730153 0.44009299"/>
<atom coord="0.27916681 0.89763820 0.90371573"/>
<atom coord="0.72083308 0.10236152 0.09628433"/>
<atom coord="0.22086546 0.39763464 0.59634741"/>
<atom coord="0.77913485 0.60236527 0.40365268"/>
<atom coord="0.35805456 0.80096287 0.81857485"/>
<atom coord="0.64194546 0.19903695 0.18142524"/>
<atom coord="0.14188542 0.30091556 0.68143485"/>
<atom coord="0.85811475 0.69908448 0.31856522"/>
<atom coord="0.48113263 0.91705427 0.76597746"/>
<atom coord="0.51886762 0.08294540 0.23402275"/>
<atom coord="0.01875659 0.41697214 0.73398372"/>
<atom coord="0.98124319 0.58302767 0.26601610"/>
<atom coord="0.52345130 0.13311247 0.80513655"/>
<atom coord="0.47654871 0.86688736 0.19486349"/>
<atom coord="0.97644175 0.63307251 0.69482952"/>
<atom coord="0.02355825 0.36692742 0.30517043"/>
<atom coord="0.44597929 0.22824278 0.89092459"/>
<atom coord="0.55402051 0.77175702 0.10907536"/>
<atom coord="0.05399747 0.72822321 0.60908048"/>
<atom coord="0.94600277 0.27177669 0.39091959"/>
</species>

<species speciesfile="N.xml" rmt="1.18">
<atom coord="0.24069109 0.21546173 0.02717112"/>
<atom coord="0.75930691 0.78454046 0.97282733"/>
<atom coord="0.25940505 0.71549655 0.47290306"/>
<atom coord="0.74059705 0.28450582 0.52709855"/>
<atom coord="0.55695070 0.82807456 0.68064169"/>
<atom coord="0.44304964 0.17192495 0.31935861"/>
<atom coord="0.94287390 0.32797497 0.81929303"/>
<atom coord="0.05712563 0.67202460 0.18070661"/>
</species>

<species speciesfile="O.xml" rmt="1.18">
<atom coord="0.13149850 0.11356132 0.07219430"/>
<atom coord="0.86850278 0.88643846 0.92780666"/>
<atom coord="0.36856860 0.61357494 0.42787370"/>
<atom coord="0.63142993 0.38642478 0.57212518"/>
<atom coord="0.28057641 0.40969073 0.05609655"/>
<atom coord="0.71942515 0.59030873 0.94390414"/>
<atom coord="0.21961446 0.90981176 0.44403726"/>
<atom coord="0.78038353 0.09018742 0.55596172"/>
</species>
<species speciesfile="H.xml" rmt="0.6">
<atom coord="0.53703101 0.66969878 0.65414322"/>
<atom coord="0.46296878 0.33030105 0.34585681"/>
<atom coord="0.96282406 0.16961581 0.84580521"/>
<atom coord="0.03717605 0.83038409 0.15419471"/>
<atom coord="0.63841032 0.92117322 0.64018288"/>
<atom coord="0.36158961 0.07882643 0.35981713"/>
<atom coord="0.86136904 0.42103949 0.85971976"/>
<atom coord="0.13863077 0.57896016 0.14028007"/>
<atom coord="0.18278113 0.81016826 0.94174334"/>
<atom coord="0.81721858 0.18983166 0.05825659"/>
<atom coord="0.31732949 0.31022523 0.55837885"/>
<atom coord="0.68267111 0.68977502 0.44162143"/>
<atom coord="0.32471970 0.63518284 0.78992572"/>
<atom coord="0.67528041 0.36481693 0.21007439"/>
<atom coord="0.17519743 0.13512682 0.71009471"/>
<atom coord="0.82480267 0.86487321 0.28990537"/>
<atom coord="0.61792355 0.22267352 0.76579570"/>
<atom coord="0.38207648 0.77732634 0.23420438"/>
<atom coord="0.88205243 0.72270296 0.73422806"/>
<atom coord="0.11794742 0.27729687 0.26577175"/>
<atom coord="0.48103484 0.39149257 0.92149143"/>
<atom coord="0.51896466 0.60850753 0.07850832"/>
<atom coord="0.01900057 0.89150889 0.57852053"/>
<atom coord="0.98099999 0.10849129 0.42147972"/>
</species>

</structure>
<groundstate
lmaxvr="10"
lmaxapw="10"
lmaxmat="10"
rgkmax="2.7"
ngridk="3 3 2"
maxscl="250"
nempty="30"
gmaxvr="14">
</groundstate>
</input>

Dear Hajar,

I think your convergence issues are related to the xc functional you are using, which is the default one for exciting, i.e. PBEsol. This functional is designed for bulk crystals rather than for molecular materials. For this reason, I suggest you to switch to LDA, which works perfectly for molecules and organic crystals

If you have some good reasons to be willing to stick to GGA, you need to increase significantly (by a factor 2 or more) the value of gmaxvr.

Best regards,
Caterina Cocchi
(exciting team)

Thank you so much dear Caterina for your guidance.
The best for you

Hajar

If it suits you, LDA is indeed a good option. Typically, LDA does not bring any convergence issues.
If you insist on using GGA, increasing gmaxvr significantly often helps. However, I have played a bit with your example, and calculations would not converge even at very large gmaxvr values. But when I have set mixerswitch="2" (at low a gmaxvr value), the calculation has converged without a problem.

Thanks a lot dear Andris

I increased lmaxvr, lmaxmat and lmaxapw to 12 and gmaxvr to 16. With these options, the scf was converged.
Now, I will set mixerswitch="2" and rerun scf. In addition, I will test my calculation by setting xctype to LDA.

The best for you.
Hajar