Dimer Method

Overview

The Dimer method is a minimum-mode following algorithm for locating first-order saddle points on the potential energy surface. Unlike path-based methods such as NEB or String, the Dimer method requires only a single starting structure and does not need knowledge of the product geometry.

The method works by rotating a trial direction toward the lowest curvature mode of the PES, then translating the structure along that mode. The current runtime uses calculator Hessian-vector products through get_hvp() for this rotation/translation loop, so Dimer requires a model backend that exposes that capability.

Parameters

Parameter	Type	Default	Description
`use_hvp`	bool	False	Compatibility flag. The current Dimer execution path requires calculator `get_hvp()` support regardless of this setting.
`delta`	float	0.005	Legacy finite-difference displacement. It is not used by the current `get_hvp()`-based execution path.
`rot_max_iter`	int	5	Maximum rotation iterations per translation step.
`rot_alpha`	float	0.5	Rotation step scaling factor.
`rot_f_max_th`	float	1.0e-3	Max force threshold for rotation convergence.
`rot_f_rms_th`	float	5.0e-4	RMS force threshold for rotation convergence.
`step0`	float	0.2	Initial translation step size.
`step_max`	float	0.15	Maximum translation step size.
`trust_radius`	float	0.15	Trust radius for translation.
`f_max_th`	float	5.0e-3	Max force convergence threshold for overall dimer.
`f_rms_th`	float	1.0e-3	RMS force convergence threshold.
`kappa_to_flip`	float	0.0	Curvature threshold for mode flipping.
`max_iter`	int	200	Maximum dimer translation iterations.
`use_mass_weight`	bool	False	Use mass-weighted coordinates.
`remove_rigid`	bool	True	Remove rigid body translations and rotations.
`n_init`	str	"random"	Initial dimer direction: "random", "force", or "given".
`n_given`	ndarray	None	User-supplied initial dimer direction vector.
`save_traj`	bool	True	Save trajectory of dimer steps.
`save_metrics`	bool	True	Save convergence metrics to output.

Input Example

Checked-in MAPLE example: examples/ts/dimer/inp1.inp. Dimer is single-ended, so the input is one starting geometry rather than reactant/product endpoints.

#model=ANI-1xnr
#ts(method=dimer)
#device=gpu0

C    0.8773307780   0.7831579390  -0.1629806432
H    0.6909163982   1.5540583811  -0.9219751886
O    0.5296065367   0.9762287325   1.1234790578
C    1.3488671845  -0.5563869274  -0.1923254310
H    0.7828987178  -0.2429788233   1.0663324170
H    1.0764966981  -1.2943318365  -0.9583627957
H    2.2951253096  -0.7293808119   0.3397718379

Important

Dimer is single-ended, but it is not model-agnostic: the active calculator must implement get_hvp(). If a selected model does not provide Hessian-vector products, choose P-RFO/NEB/String instead or switch to a compatible backend.

Output

*_dimer_ts.xyz — The final converged transition state geometry.
*_dimer_traj.xyz — Multi-frame XYZ trajectory file containing all intermediate structures generated during the search from the initial guess to the saddle point.

Visualization

The Dimer trajectory visualization uses *_dimer_traj.xyz, while the final TS panel uses *_dimer_ts.xyz. Because Dimer is single-ended, the trajectory shows the search process rather than a reactant-to-product MEP.

Dimer transition state search trajectory with energy trace — Fig. 1 — Dimer single-ended TS search trajectory with energy trace.

Dimer final transition state geometry — Fig. 2 — Dimer final TS geometry.

When to Use

The Dimer method is the right choice when:

The product is unknown. You want to explore what reactions are possible from a given structure without knowing the endpoint geometry in advance.
Exploring the PES landscape. You want to systematically discover nearby saddle points from a minimum, for example to enumerate possible decomposition pathways or rearrangement channels.
Single-ended search is preferred. You have a reasonable guess for the TS geometry (e.g., from chemical intuition or a constrained scan) but do not have a product structure to use with NEB or String.
The reaction path is not well-defined. When the reaction involves complex conformational changes where defining a clear product is difficult, the Dimer method can find the nearest saddle point without requiring path construction.