Why Choose Sunny?

Powerful and easy to use

Feature highlights include:

• Ability to specify a crystal from a .cif file, from its spacegroup number and Wyckoffs, or from a full chemical cell with automatically inferred symmetry operations. Magnetic structures can be read from .mcif files.
• Interactive visualization of 3D crystals and magnetic structures.
• Symmetry analysis to determine allowed anisotropies and interaction terms, and to propagate them by symmetry equivalence.
• Single-ion anisotropy can be specified using either Stevens operators or spin polynomials. Arbitrary coupling between spin multipoles is also supported. Classical-to-quantum renormalization factors enhance fidelity.
• Fast optimization of magnetic structures using supercells or the propagation vector formalism.
• Statistical sampling of spins in thermal equilibrium using Langevin dynamics or local Monte Carlo updates. Advanced Monte Carlo methods, such as parallel tempering, for simulations of classical spin liquids and frustrated magnetism.
• Classical dynamics of spin dipoles and its generalization to SU(N) coherent states. One can sample dynamical correlations at finite temperature. The CP² skyrmion example illustrates a highly non-equilibrium quench process that depends crucially on spin quadrupole degrees of freedom.
• Generalized linear spin wave theory (LSWT) for low-temperature spin dynamics. Special support is provided for efficient calculations on incommensurate spiral phases and on very large magnetic cells. The FeI₂ example showcases LSWT with multi-flavor bosons. The disordered system example demonstrates acceleration for large system sizes.
• Dipole-dipole interactions with full Ewald summation, as illustrated in the pyrochlore LSWT example. Dipole-dipole interactions in classical dynamics are accelerated with the fast Fourier transform (FFT).
• Tools for comparing to experimental data: form factors, custom spin contractions, averaging over powder and domain orientations, etc.
• Programmatic interface in the Julia language for full flexibility and performance.

But still evolving:

• Sunny does not yet have GPU acceleration of classical spin dynamics. An alternative here might be Vampire.

Advanced theory made accessible

Sunny is also a platform for disseminating foundational advances in quantum magnetism and computational methods. The theory of SU(N) coherent states offers a group theoretic framework to formulate alternative classical limits of a microscopic quantum model. New algorithms enable highly efficient simulation of spin-multipoles and beyond. For reasons not fully understood, such classical limits can be remarkably accurate at finite temperatures when used in conjunction with appropriate renormalization schemes. The SU(N) picture also suggests new geometric interpretations of quantum spin. This leads to a deeper understanding of existing renormalization schemes for traditional spin wave theory, and suggests a whole landscape in which to search for novel topological states. Ongoing Sunny research aims to incorporate more quantum entanglement into the classical picture. Local units of strongly entangled spins will soon be supported and show great promise for cases like dimerized ladders. Longer term, Sunny also aims to include perturbative corrections beyond linear spin wave theory, as well as a non-perturbative treatment of quantum bound states.