GMX-ROTACF(1)

💖 GMX-ROTACF(1) 💖

okay bestie, so this is gmx-rotacf from the GROMACS manual, and it's literally the moment your molecules get their little spinny glow-up. let's go 🎀

---

✨ NAME ✨

gmx-rotacf — Calculate the rotational correlation function for molecules

---

🌸 SYNOPSIS 🌸

gmx rotacf [-f [<.xtc/.trr/...>]] [-s [<.tpr>]] [-n [<.ndx>]]
           [-o [<.xvg>]] [-b <time>] [-e <time>] [-dt <time>]
           [-[no]w] [-xvg <enum>] [-[no]d] [-[no]aver]
           [-acflen <int>] [-[no]normalize] [-P <enum>]
           [-fitfn <enum>] [-beginfit <real>] [-endfit <real>]

---

🦋 DESCRIPTION 🦋

so gmx rotacf calculates the rotational correlation function for molecules. that's the whole vibe. 💅

but here's the tea: you have to give it atom triplets (i,j,k) in the index file, and those little guys define two vectors, ij and jk. then it calculates the rotational ACF as the autocorrelation function of the vector

n = ij x jk

aka the cross product of those two vectors. and no cap — since three atoms span a plane, the order of the three atoms does not matter. it's giving "any order works, we're not picky". 🫶

optionally, if you invoke the -d switch, you can calculate the rotational correlation function for linear molecules by specifying atom pairs (i,j) in the index file. iconic behaviour honestly.

🌈 EXAMPLES 🌈

gmx rotacf -P 1 -nparm 2 -fft -n index -o rotacf-x-P1 -fa expfit-x-P1
-beginfit 2.5 -endfit 20.0

so what this is doing: it'll calculate the rotational correlation function using a first order Legendre polynomial of the angle of a vector defined by the index file. then the correlation function gets fitted from 2.5 ps until 20.0 ps to a two-parameter exponential. slay. ✨

---

💅 OPTIONS 💅

🍓 Options to specify input files:

• -f [<.xtc/.trr/...>] (traj.xtc) — Trajectory: xtc trr cpt gro g96 pdb tng h5md
• -s [<.tpr>] (topol.tpr) — Portable xdr run input file
• -n [<.ndx>] (index.ndx) — Index file

🎀 Options to specify output files:

• -o [<.xvg>] (rotacf.xvg) — xvgr/xmgr file

💜 Other options:

• -b <time> (0) — Time of first frame to read from trajectory (default unit ps)
• -e <time> (0) — Time of last frame to read from trajectory (default unit ps)
• -dt <time> (0) — Only use frame when t MOD dt = first time (default unit ps)
• -[no]w (no) — View output .xvg, .xpm, .eps and .pdb files
• -xvg <enum> (xmgrace) — xvg plot formatting: xmgrace, xmgr, none
• -[no]d (no) — Use index doublets (vectors) for correlation function instead of triplets (planes)
• -[no]aver (yes) — Average over molecules
• -acflen <int> (-1) — Length of the ACF, default is half the number of frames
• -[no]normalize (yes) — Normalize ACF
• -P <enum> (0) — Order of Legendre polynomial for ACF (0 indicates none): 0, 1, 2, 3
• -fitfn <enum> (none) — Fit function: none, exp, aexp, exp_exp, exp5, exp7, exp9
• -beginfit <real> (0) — Time where to begin the exponential fit of the correlation function
• -endfit <real> (-1) — Time where to end the exponential fit of the correlation function, -1 is until the end

---

👑 SEE ALSO 👑

gmx(1)

More information about GROMACS is available at <‐ http://www.gromacs.org/>.

---

🧚 COPYRIGHT 🧚

2026, GROMACS development team

---

okay that's the whole page bestie, you ate that 💖 no more notes, go calculate some correlation functions and be the main character of your own trajectory. fr fr 🦋

2026.4 — Oct 02, 2026 🌈 mwah, love u, bye!! 💅✨