CASPER

Conformer-Averaged Surface Property Encoded Representation — a tunable 3D molecular descriptor.

CASPER turns a molecule into a fixed-length feature vector by:

1. generating ETKDG conformers (no force-field minimisation needed by default),
2. building a van-der-Waals dot surface for each conformer,
3. colouring each surface point by an atomic property (partial charge, logP, molar refractivity, TPSA, H-bond donor/acceptor, …),
4. encoding the coloured surface as a property histogram (a 3D generalisation of MOE-style VSA descriptors) and/or a density-invariant spatial autocorrelation (VolSurf-flavoured), and
5. pooling across conformers.

Unlike 2D VSA descriptors, CASPER is built on real 3D conformer surfaces, and unlike single-conformer VolSurf it averages over a conformer ensemble. Every step is exposed as a tunable parameter, and the per-conformer "bag" can be returned un-pooled for multi-instance / key-instance learning.

Install

pip install casper-descriptor
# with visualisation extras:
pip install "casper-descriptor[viz]"

Core dependencies: numpy, rdkit, scikit-learn.

Quick start

import casper

# one molecule -> one fixed-length vector (default config)
v = casper.featurize("CC(=O)Oc1ccccc1C(=O)O")

# tune the construction
cfg = casper.CasperConfig(
    n_confs=10,
    properties=("gasteiger", "abs_charge", "logp", "mr", "tpsa"),
    encoding=("hist", "autocorr"),
    n_bins=12, autocorr_bins=8, autocorr_max_dist=16.0,
    conf_pool=("mean", "max"),
)
v = casper.featurize("CCO", cfg)

# names carry full provenance: e.g. "mean:gasteiger|ac[0.0,2.0)A"
v, names = casper.featurize("CCO", cfg, return_names=True)

# batch, parallel across molecules
X = casper.featurize_many(smiles_list, cfg, n_jobs=-1)

# sklearn transformer (drops into Pipeline / GridSearchCV)
from casper import CasperFeaturizer
ft = CasperFeaturizer(n_confs=10, density=16, encoding=("hist", "autocorr"))
X = ft.fit_transform(smiles_list)

Multi-instance learning (un-pooled bag)

For key-instance detection, get the per-conformer instances without pooling:

bag, conformer_ids, names = casper.featurize_bag("CCO", cfg)
# bag: (K, d) array, one CASPER vector per conformer; K varies per molecule
# conformer_ids: trace a flagged key-instance back to its 3D geometry (deterministic for a fixed seed)

bags, names = casper.featurize_bags(smiles_list, cfg)   # ragged list of (K_i, d)

casper.featurize(...) is exactly pool() applied to this bag.

Feature visualization

When a model flags a CASPER feature as important, you can see what surface region it measures — every feature name carries full provenance back to the surface.

import casper
cfg = casper.CasperConfig(properties=("gasteiger",), encoding=("hist", "autocorr"))

# write a static PNG (matplotlib) and/or interactive HTML (py3Dmol)
casper.explain_feature("CC(=O)Oc1ccccc1C(=O)O", "mean:gasteiger|ac[2.0,4.0)A",
                       cfg, png="feature.png", html="feature.html")

# in a notebook, omit png/html for an inline interactive py3Dmol view
casper.explain_feature("CCO", "mean:gasteiger|hist[0.17,0.25)", cfg)

• Histogram bins highlight the surface points whose property falls in the bin (the highlighted area provably equals the feature's value).
• Autocorrelation bins show either a per-point contribution score (autocorr_mode="contribution", default) or the literal contributing point-pairs at that separation (autocorr_mode="pairs").

Requires the viz extra: pip install "casper-descriptor[viz]".

Key parameters (CasperConfig)

parameter	default	what it does
n_confs	10	ETKDG conformers per molecule
optimize	"none"	"none" (raw ETKDG, fast) / "mmff" / "uff"
properties	("gasteiger","logp","mr")	which atomic properties colour the surface
probe	0.0	0.0 = VdW surface; 1.4 = water-accessible
density	16	surface dots per atom (knee of the accuracy/cost curve; cost is ~quadratic via autocorr)
encoding	("hist",)	"hist" and/or "autocorr"
n_bins	12	histogram bins per property
autocorr_bins, autocorr_max_dist	8, 12.0	distance bins / radial extent for autocorrelation
autocorr_normalize	True	density-invariant mean-per-bin (recommended) vs legacy area-weighted sum
autocorr_range	None	per-property (max_dist, n_bins) override
conf_pool	("mean",)	mean/max/min/std/boltzmann, concatenated

Add your own colouring:

import numpy as np
casper.register_property("my_prop", lambda mol: np.array([...]), (lo, hi))

Optional: kallisto / Jazzy properties

Five extra per-atom colourings derived from Jazzy (kallisto EEQ charges) add signal orthogonal to the built-ins — a different charge model (eeq), a real charge-dependent dynamic polarisability (alp, unlike a per-element constant), and continuous H-bond strengths (sa acceptor, sdc/sdx donor) rather than binary flags:

import casper.jazzy_properties        # registers: eeq, alp, sa, sdc, sdx
cfg = casper.CasperConfig(properties=("gasteiger", "eeq", "alp", "sa", "sdc", "sdx"))
v = casper.featurize("CC(=O)Nc1ccc(O)cc1", cfg)

They are computed once per molecule on CASPER's own geometry (cached), so all five cost roughly one kallisto evaluation. Requires pip install "casper-descriptor[jazzy]" (note: jazzy pins numpy<2).

Notes

• density=16 is the default because surface cost is roughly quadratic (the autocorrelation is O(points²)) and accuracy plateaus there; lower (12) degrades generalisation, higher (24/32) costs more for no measurable gain.
• The normalized autocorrelation is density-invariant, so changing density does not silently rescale features.
• Conformer count K is data-dependent (ETKDG + RMS pruning). Set prune_rms=0 for a fixed K.

License

MIT