$$\renewcommand{\AA}{\text{Å}}$$

# pair_style lepton command¶

Accelerator Variants: lepton/omp, lepton/coul/comp

## Syntax¶

pair_style style args
• style = lepton or lepton/coul

• args = list of arguments for a particular style

lepton args = cutoff
cutoff = global cutoff for the interactions (distance units)
lepton/coul args = cutoff keyword
cutoff = global cutoff for the interactions (distance units)
zero or more keywords may be appended
keyword = ewald or pppm or msm or dispersion or tip4p

## Examples¶

pair_style lepton 2.5

pair_coeff  * *  "k*((r-r0)^2*step(r0-r)); k=200; r0=1.5" 2.0
pair_coeff  1 2  "4.0*eps*((sig/r)^12 - (sig/r)^6);eps=1.0;sig=1.0" 1.12246204830937
pair_coeff  2 2  "eps*(2.0*(sig/r)^9 - 3.0*(sig/r)^6);eps=1.0;sig=1.0"
pair_coeff  1 3  "zbl(13,6,r)"
pair_coeff  3 3  "(1.0-switch)*zbl(6,6,r)-switch*4.0*eps*((sig/r)^6);switch=0.5*(tanh(10.0*(r-sig))+1.0);eps=0.05;sig=3.20723"

pair_style lepton/coul 2.5
pair_coeff 1 1 "qi*qj/r" 4.0
pair_coeff 1 2 "lj+coul; lj=4.0*eps*((sig/r)^12 - (sig/r)^6); eps=1.0; sig=1.0; coul=qi*qj/r"

pair_style lepton/coul 2.5 pppm
kspace_style pppm 1.0e-4
pair_coeff 1 1 "qi*qj/r*erfc(alpha*r); alpha=1.067"

## Description¶

New in version 8Feb2023.

Pair styles lepton and lepton/coul compute pairwise interactions between particles which depend solely on the distance and have a cutoff. The potential function must be provided as an expression string using “r” as the distance variable. With pair style lepton/coul one may additionally reference the charges of the two atoms of the pair with “qi” and “qj”, respectively. Note that further constants in the expression can be defined in the same string as additional expressions separated by semi-colons as shown in the examples above.

The expression “200.0*(r-1.5)^2” represents a harmonic potential around the pairwise distance $$r_0$$ of 1.5 distance units and a force constant K of 200.0 energy units:

$U_{ij} = K (r-r_0)^2$

The expression “qi*qj/r” represents a regular Coulombic potential with cutoff:

$U_{ij} = \frac{C q_i q_j}{\epsilon r} \qquad r < r_c$

The Lepton library, that the lepton pair style interfaces with, evaluates this expression string at run time to compute the pairwise energy. It also creates an analytical representation of the first derivative of this expression with respect to “r” and then uses that to compute the force between the pairs of particles within the given cutoff.

The following coefficients must be defined for each pair of atoms types via the pair_coeff command as in the examples above, or in the data file or restart files read by the read_data or read_restart commands:

• Lepton expression (energy units)

• cutoff (distance units)

The Lepton expression must be either enclosed in quotes or must not contain any whitespace so that LAMMPS recognizes it as a single keyword. More on valid Lepton expressions below. The last coefficient is optional; it allows to set the cutoff for a pair of atom types to a different value than the global cutoff.

For pair style lepton only the “lj” value of the special_bonds settings apply in case the interacting pair is also connected with a bond. The potential energy will only be added to the “evdwl” property.

For pair style lepton/coul only the “coul” value of the special_bonds settings apply in case the interacting pair is also connected with a bond. The potential energy will only be added to the “ecoul” property.

In addition to the functions listed below, both pair styles support in addition a custom “zbl(zi,zj,r)” function which computes the Ziegler-Biersack-Littmark (ZBL) screened nuclear repulsion for describing high-energy collisions between atoms. For details of the function please see the documentation for pair style zbl. The arguments of the function are the atomic numbers of atom i (zi), atom j (zj) and the distance r. Please see the examples above.

## Lepton expression syntax and features¶

Lepton supports the following operators in expressions:

 + Add - Subtract * Multiply / Divide ^ Power

The following mathematical functions are available:

 sqrt(x) Square root exp(x) Exponential log(x) Natural logarithm sin(x) Sine (angle in radians) cos(x) Cosine (angle in radians) sec(x) Secant (angle in radians) csc(x) Cosecant (angle in radians) tan(x) Tangent (angle in radians) cot(x) Cotangent (angle in radians) asin(x) Inverse sine (in radians) acos(x) Inverse cosine (in radians) atan(x) Inverse tangent (in radians) sinh(x) Hyperbolic sine cosh(x) Hyperbolic cosine tanh(x) Hyperbolic tangent erf(x) Error function erfc(x) Complementary Error function abs(x) Absolute value min(x,y) Minimum of two values max(x,y) Maximum of two values delta(x) delta(x) is 1 for x = 0, otherwise 0 step(x) step(x) is 0 for x < 0, otherwise 1

Numbers may be given in either decimal or exponential form. All of the following are valid numbers: 5, -3.1, 1e6, and 3.12e-2.

As an extension to the standard Lepton syntax, it is also possible to use LAMMPS variables in the format “v_name”. Before evaluating the expression, “v_name” will be replaced with the value of the variable “name”. This is compatible with all kinds of scalar variables, but not with vectors, arrays, local, or per-atom variables. If necessary, a custom scalar variable needs to be defined that can access the desired (single) item from a non-scalar variable. As an example, the following lines will instruct LAMMPS to ramp the force constant for a harmonic bond from 100.0 to 200.0 during the next run:

variable fconst equal ramp(100.0, 200)
bond_style lepton
bond_coeff 1 1.5 "v_fconst * (r^2)"

An expression may be followed by definitions for intermediate values that appear in the expression. A semicolon “;” is used as a delimiter between value definitions. For example, the expression:

a^2+a*b+b^2; a=a1+a2; b=b1+b2

is exactly equivalent to

(a1+a2)^2+(a1+a2)*(b1+b2)+(b1+b2)^2

The definition of an intermediate value may itself involve other intermediate values. Whitespace and quotation characters (’'’ and ‘”’) are ignored. All uses of a value must appear before that value’s definition. For efficiency reasons, the expression string is parsed, optimized, and then stored in an internal, pre-parsed representation for evaluation.

Evaluating a Lepton expression is typically between 2.5 and 5 times slower than the corresponding compiled and optimized C++ code. If additional speed or GPU acceleration (via GPU or KOKKOS) is required, the interaction can be represented as a table. Suitable table files can be created either internally using the pair_write or bond_write command or through the Python scripts in the tools/tabulate folder.

Styles with a gpu, intel, kk, omp, or opt suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed on the Accelerator packages page. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues.

These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the Build package page for more info.

You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the -suffix command-line switch when you invoke LAMMPS, or you can use the suffix command in your input script.

See the Accelerator packages page for more instructions on how to use the accelerated styles effectively.

## Mixing, shift, table, tail correction, restart, rRESPA info¶

Pair styles lepton and lepton/coul do not support mixing. Thus, expressions for all I,J pairs must be specified explicitly.

Only pair style lepton supports the pair_modify shift option for shifting the energy of the pair interaction so that it is 0 at the cutoff, pair style lepton/coul does not.

The pair_modify table options are not relevant for the these pair styles.

These pair styles do not support the pair_modify tail option for adding long-range tail corrections to energy and pressure.

These pair styles write its information to binary restart files, so pair_style and pair_coeff commands do not need to be specified in an input script that reads a restart file.

These pair styles can only be used via the pair keyword of the run_style respa command. They do not support the inner, middle, outer keywords.

## Restrictions¶

These pair styles are part of the LEPTON package and only enabled if LAMMPS was built with this package. See the Build package page for more info.

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