How to use the `diffcalc` solver#

This guide shows how to create a diffractometer with the diffcalc solver, orient a crystalline sample, and compute reciprocal-space positions. It assumes you have already installed the package.

The diffcalc solver wraps diffcalc-core. This library provides a single six-circle geometry, diffcalc_4S_2D (the psic geometry described by You 1999), with 23 operating modes.

Create a diffractometer#

import hklpy2

psic = hklpy2.creator(
    solver="diffcalc",
    geometry="diffcalc_4S_2D",
    name="psic",
)

The object psic has six real axes (mu, delta, nu, eta, chi, phi) and three pseudo axes (h, k, l).

Set the crystal lattice#

psic.add_sample(name="silicon", a=hklpy2.SI_LATTICE_PARAMETER)
psic.beam.wavelength.put(1.54)

Add orientation reflections#

Provide two reflections measured at known motor positions:

r1 = psic.add_reflection(
    pseudos={"h": 4, "k": 0, "l": 0},
    reals={"mu": 0, "delta": 69.0966, "nu": 0, "eta": 34.5483, "chi": 0, "phi": 0},
    wavelength=1.54,
    name="r1",
)
r2 = psic.add_reflection(
    pseudos={"h": 0, "k": 4, "l": 0},
    reals={"mu": 0, "delta": 69.0966, "nu": 0, "eta": 34.5483, "chi": 0, "phi": 90},
    wavelength=1.54,
    name="r2",
)

Calculate the UB matrix#

psic.core.calc_UB(r1, r2)

Choose an operating mode#

The default mode is 4S+2D bisect_eta_fixed nu_fixed (vertical bisector: eta = delta/2, eta=0, nu=0). To change it:

psic.core.mode = "4S+2D mu_chi_phi_fixed"

See diffcalc solver for the full list of 23 modes.

Compute motor positions (forward)#

psic.forward(4, 0, 0)

This returns a single chosen motor-position solution for the given (h, k, l) (an Hklpy2DiffractometerRealPos). The underlying solver’s forward() may return multiple solutions; the diffractometer picks one according to the policy assigned to psic._forward_solution (defaults to hklpy2.utils.pick_first_solution()). The complete list of solutions can be returned from psic.core.forward((4, 0, 0)). See the upstream hklpy2 guide How to Choose the Default forward() Solution for details.

Tip

The two call shapes differ: psic.forward(4, 0, 0) takes h, k, l as separate positional arguments, while psic.core.forward((4, 0, 0)) takes a single sequence (tuple / list / ndarray) or dict (e.g. {"h": 4, "k": 0, "l": 0}).

Compute (h, k, l) from motor positions (inverse)#

psic.inverse(psic.real_position)

This returns the (h, k, l) values computed from the supplied motor positions. psic.real_position is the current readout of all real axes; pass a different set of values to compute (h, k, l) at a hypothetical position instead.

Available geometries at a glance#

Geometry	Real axes	Modes	Default mode
diffcalc_4S_2D	mu, delta, nu, eta, chi, phi	23	4S+2D bisect_eta_fixed nu_fixed

How to use the diffcalc solver#