VMD Integration¶

FastSASA ships a fastsasa Tcl command for VMD that behaves like a GPU-accelerated alternative to VMD's built-in measure sasa: same selection semantics, same radii by default, plus whole-trajectory calculation in one call, an animated accessible-surface visualization, and an on-screen SASA counter.

Setup¶

Add the package to VMD's plugin path in ~/.vmdrc:

lappend auto_path /path/to/fastsasa/integration/vmd

and make sure the fastsasa binary is on PATH, or point at it explicitly with the FASTSASA_EXE environment variable or the -exe option.

Usage¶

In any VMD session (GUI or vmd -dispdev text):

package require fastsasa

set sel [atomselect top protein]

# Total SASA of the current frame (VMD radii, like measure sasa):
set area [fastsasa -sel $sel]

# Every frame of the loaded trajectory, streamed through the GPU engine:
set areas [fastsasa -sel $sel -frames all]

# measure sasa -restrict equivalent (selA reported, everything occludes):
set all [atomselect top all]
set chainA_area [fastsasa -sel $all -restrict [atomselect top "chain A"]]

# Per-atom SASA into a Tcl list, and stored on the atoms if you like:
fastsasa -sel $sel -peratom areas
$sel set user $areas

# Accessible surface points (measure sasa -points equivalent):
fastsasa -sel $sel -points pts

# Draw the accessible surface and show the on-screen counter:
fastsasa -sel $sel -visualize 1 -color yellow

# Whole trajectory with a synced animated surface and counter:
set areas [fastsasa -sel $sel -frames all -visualize 1]

# Huge trajectory on disk without loading it into VMD:
set areas [fastsasa -topfile sys.psf -trajfile run.dcd -filter protein]

Options¶

-probe R — probe radius, default 1.4.
-samples N — sphere points per atom, default 500 (matching measure sasa; FastSASA outside VMD defaults to 100).
-frames now|all — current frame, or every loaded frame as a per-frame list.
-restrict $sel2 — report only these atoms; everything in -sel still occludes. -frames now only.
-peratom var — store the per-atom SASA list. -frames now only.
-points var — store accessible surface points as {x y z} lists. -frames now only.
-visualize 0|1 — default off; the one switch for all visuals. With -frames now it draws the current frame's point cloud, with -frames all it loads the animated surface track; both show the counter.
-track 0|1 — the animated surface as its own molecule (default: on with -visualize 1 -frames all).
-track-samples N — surface density for the track, independent of -samples (default: -samples/10).
-hud 0|1 — on-screen SASA counter (default: on with -visualize 1 or -track 1).
-color name — drawing color, default yellow.
-radii vmd|protor — radius source, default vmd (the molecule's own per-atom radii).
-backend auto|vulkan|cuda|cpu and -precision fp32|fp64 — default fp32 inside VMD, matching VMD's single-precision arithmetic.
-threads N — CPU thread count passed through to the CLI's --threads (ignored on GPU backends).
-exe path — explicit path to the fastsasa binary.
-topfile F -trajfile F -filter EXPR — analyze files on disk directly, without loading them into VMD.
-clear 1 — remove the molecule's FastSASA cloud/track and counter; runs no calculation.

Mapping to measure sasa and the CLI:

VMD `measure sasa`	`fastsasa` wrapper	FastSASA CLI
`<srad>`	`-probe R`	`--probe-radius R`
`-samples N` (default 500)	`-samples N` (default 500)	`--resolution N` (default 100)
`-restrict $sel2`	`-restrict $sel2`	`--select`
molecule radii	`-radii vmd` (default)	generated `--config-file`
n/a	`-radii protor`	bundled ProtOr table
n/a	`-frames now\\|all`	`--frames`
n/a	`-backend`, `-precision`	`--backend`, `--precision`

Defaults and comparability with `measure sasa`¶

Inside VMD the wrapper defaults to -radii vmd, -samples 500, and -precision fp32, matching measure sasa's radii, sampling density, and precision class, so bare fastsasa and bare measure sasa calls are directly comparable. (VMD stores coordinates and accumulates SASA in single precision; FastSASA's fp32 mode still counts exposures exactly and reduces in FP64, so it is the more accurate of the two.) Outside VMD, FastSASA defaults to ProtOr radii, 100 samples, and FP64; pass -radii protor -samples 100 -precision fp64 in VMD to reproduce the native CLI/Python defaults exactly.

Values differ slightly from measure sasa by design. Both tools use deterministic sphere sampling, but different point sets (FastSASA uses a Fibonacci sphere), so at a finite sample count the two quadratures carry different discretization bias. With matched radii and sample counts, totals typically agree within a fraction of a percent, and the gap shrinks as -samples grows. Use the same radii, probe radius, atom set, and sample count when comparing results from different programs.

The surface points are generated by the same exposure test as the SASA calculation, so the point cloud is exactly the reported surface: per atom, 4 * pi * r^2 * points / samples equals the atom's SASA.

Frames and indexing¶

With -frames all, element i of the returned list is VMD frame i of the source molecule — the same index the animation slider, the surface track, and the on-screen counter use. Note that when a topology file contributes a coordinate frame (e.g. mol new top.pdb followed by mol addfile traj.dcd), that PDB frame is VMD frame 0, so the list has one more element than the DCD has frames. Run the CLI on the trajectory file directly (or use -topfile/-trajfile) if you want DCD-only indexing.

Visualization¶

Nothing is drawn unless you ask: bare calls return numbers only. -visualize 1 is the single switch — with -frames now it draws the current frame's points, with -frames all it loads the animated surface track, and in both cases it shows the on-screen counter. Once something is drawn it stays until you replace it with another -visualize 1 call or remove it:

fastsasa -sel $sel -clear 1

Three things to know when visualizing:

A -frames now -visualize 1 cloud belongs to one frame — whatever frame was current when the call ran. Drawn points are static graphics and do not follow animation; re-run after moving to a new frame, or use -frames all -visualize 1 for an animated surface.
Everything in -sel occludes. With -sel [atomselect top all] on a solvated or membrane system, water and lipids bury most of the protein surface and the points there correctly disappear. Select the molecule set that should count, e.g. protein.
Buried surface has no points by definition — interfaces and the core show gaps because their accessible area is zero. The cloud sits on the probe-center surface, about a probe radius above the van der Waals surface.

Surface track: animated, toggleable¶

fastsasa -sel $sel -frames all -visualize 1 (or -track 1) computes the accessible surface of every frame alongside the per-frame SASA values and loads it as a separate molecule named FastSASA surface (<source>), one frame per trajectory frame, drawn with the Points representation. It is an ordinary molecule, so the usual controls apply:

set areas [fastsasa -sel $sel -frames all -visualize 1 -color yellow]
set track $::FastSASA::track([$sel molid])   ;# molecule id of the track
mol off $track                              ;# hide (or click D in the molecule list)
mol on $track                               ;# show again
mol modstyle 0 $track Points 4.0            ;# bigger dots
mol modcolor 0 $track ColorID 1             ;# recolor
mol delete $track                           ;# remove

Each call replaces the previous track for that source molecule. The track molecule is kept out of VMD's own animation and its frame is driven from the source molecule's vmd_frame callback, so the cloud always shows exactly the frame the protein is showing, whether the trajectory is playing or being scrubbed.

Track density and cost. The reported areas always use the full -samples density; the drawn cloud defaults to a tenth of it (-track-samples), because surface-point extraction and loading dominate the call at full density. The default track adds a few seconds to a -frames all call; a full-density track can take an order of magnitude longer and produce hundreds of thousands of points per frame. Pass -track-samples 500 if you want the dense cloud anyway. The reported totals (-samples) and the track (-track-samples) are extracted in one CLI pass. If rotating the scene stutters, a smaller mol modstyle ... Points 1.0 point size and disabling depth cueing help.

-track surface extraction uses the Vulkan kernel when available. CPU and CUDA backends use the threaded CPU surface-point path. Single-frame -points/-visualize also uses the CPU path.

On-screen SASA counter¶

-hud comes on with -visualize 1 (or -track 1) — a small "SASA: 1234.5 Å²" label pinned to the top-right corner of the screen. Pass -hud 1 on its own for the counter without any surface drawing, or -hud 0 to suppress it while visualizing.

With -frames now the counter shows that one call's total. With -frames all it follows the currently displayed frame as the trajectory plays or is scrubbed — no recomputation, just a lookup into the per-frame list the call already returned. -hud needs Tk, which ships with VMD; under vmd -dispdev text with no display it prints one warning and is otherwise a no-op, so it is safe in scripts that might run headless.

Performance notes¶

Prefer one -frames all call over a per-frame Tcl loop: each separate call pays process startup and (on GPU backends) context creation.
For unavoidable single-frame calls, -backend cpu skips GPU context creation and is usually the fastest end-to-end option.
Temporary topology/trajectory exports go to RAM-backed storage when available (/dev/shm), honor TMPDIR, and are removed on completion and on error. They can be large for big systems.

Caveats¶

The wrapper is Tcl 8.5-compatible, matching the Tcl bundled with VMD 1.9.3/1.9.4.
FastSASA builds natively on Windows (fastsasa.exe, CPU and Vulkan), and the wrapper can point FASTSASA_EXE at it, but the combination inside Windows VMD is untested.