# Notation of spin Hamiltonian#

The notation, which is considered to be the standard for the RAD-tools:

$H = -\sum_{i,j} \hat{\boldsymbol{S}}_i \cdot \boldsymbol{J}_{i,j} \hat{\boldsymbol{S}}_j$

It has double-counting, spins are not normalized, and the exchange parameter is positive for ferromagnetic order.

Note

The "standard" here does not mean that the SpinHamiltonian always has this notation. For example when SpinHamiltonian is read from TB2J file (load_tb2j_model()) it has the notation of TB2J.

Notation of the spin Hamiltonian is defined with two properties and the numerical factor before the sum. The properties are:

• double_counting

Whether both pairs $$(i, j)$$ and $$(j, i)$$ are included in the sum.

If double counting is avoided it is usually indicated under the sum sign:

$\sum_{i < j} \text{ or } \sum_{i > j}$

When double counting is implied there are no indication under the sum sign:

$\sum_{ij}$

However, some authors are using

$\sum_{<ij>}$

as an indication of the avoided double counting, which we find confusing and discourage you to use it in this way. In textbooks this indication usually implies that only near-neighbors are included in the sum, which is not the same as avoided double counting (consecutively $$\sum_{<<ij>>}$$ is used to indicate next-nearest neighbors and so on).

Note

Indication

$\sum_{i \ne j}$

Does not mean that double counting is avoided. It rather say that no exchange of the atom with itself is present.

• spin_normalized

$$\boldsymbol{S}_i$$ in the spin Hamiltonian is viewed as a spin operator or as a classical spin vector. If it is normalised to $$1$$, then exchange parameter absorbs the factor $$\vert\boldsymbol{S}_i\vert \vert\boldsymbol{S}_j\vert$$.

Numerical factor in general can be arbitrary, but in practice only three cases are met:

• Factor $$\dfrac{1}{2}$$

Factor $$1/2$$ is sometimes included in order to compensate for the double counting. If it is included, then it is simply written before the sum sign. It is usually used when double counting is present.

• Factor $$2$$

Factor $$2$$ is sometimes included in order to account for double counting. If it is included, then it is simply written before the sum sign. It is usually used when double counting is avoided.

• Minus sign

Whether the minus sign is included in the Hamiltonian.

Caution

We would like to note that not all authors are thoughtful with the definition of the Hamiltonian, therefore, additional care is required when reading the literature.

RAD-tools utilizes those properties in order to define the notation of the spin Hamiltonian. During the creation of the SpinHamiltonian object the notation is deliberately not defined, because it depends on your interpretation. Therefore, the notation has to be defined explicitly by you. If the notation is not defined, and you are trying to use the properties and methods that rely on it, then the NotationError is raised. See Examples for the usage.

## Conversion of the notation#

In the figure below we illustrate the problem of the notation's convention. The picture describes the whole notation tree, if no minus sign is considered present (SpinHamiltonian.factor is positive). Click on the picture to enlarge it.

In the picture we assume that there are only two centers $$i$$ and $$j$$, which are connected by the exchange parameter $$\boldsymbol{J}_{i,j}$$ (as displayed in the left top corner). The alignment of the spins is ferromagnetic. Then the numbers on the right side of the picture are the energies of that system in the corresponding notation under the assumption that exchange parameter is equal to $$1$$ in each notation.

These numbers provide a way for you to convert the exchange parameters from one notation to another. The conversion is based on the following principle:

Energy of the system should not depend on the choice of notation.

Note, that in the picture the systems are essentially are different since the exchange parameters are equal to one in each notation and the energies are different.

The rule for the conversion from notation one (N1) to the notation two (N2) is the following:

• Follow the notation tree for each notation. Obtain factor one $$f_1$$ and factor two $$f_2$$.

• If in the notation one (N1) minus sign is written (i.e. positive exchange parameter means ferromagnetic alignment), multiply the factor $$f_1$$ by $$-1$$.

• If in the notation two (N2) minus sign is written (i.e. positive exchange parameter means ferromagnetic alignment), multiply the factor $$f_2$$ by $$-1$$.

• compute conversion factor $$f$$ for the exchange parameters as

$f = \dfrac{f_1}{f_2}$
• Multiply exchange parameters in the notation one (N1) by the conversion factor $$f$$.

The result are the exchange parameters in the notation two (N2):

$J_{N2} = \dfrac{f_1}{f_2} J_{N1}$

SpinHamiltonian does this automatically for you, when you are trying to set the SpinHamiltonian.notation property or individual properties of the notation: SpinHamiltonian.double_counting, SpinHamiltonian.spin_normalized, SpinHamiltonian.factor. See Examples for the usage. Note that when you set the notation for the very first time the interpretation is set, therefore, the parameters are not changed.

## Predefined notations#

There are several predefined notations in the RAD-tools. Each predefined notation is a tuple of two bool and one float, which correspond to the three properties of the notation.

Hint

Order: (double counting, spin normalized, factor).

• Standard

(True, False, -1)

$H = -\sum_{i,j} \hat{\boldsymbol{S}}_i \cdot \boldsymbol{J}_{i,j} \cdot \hat{\boldsymbol{S}}_j$

where double counting is present ($$ij$$ and $$ji$$ are in the sum). Spin vectors are not normalized.

• TB2J

(True, True, -1)

$H = -\sum_{i,j} \hat{\boldsymbol{e}}_i \cdot \boldsymbol{J}_{i,j} \cdot \hat{\boldsymbol{e}}_j$

where double counting is present ($$ij$$ and $$ji$$ are in the sum). Spin vectors are normalized to 1.

• Vampire

(True, True, -1/2)

$H = - \dfrac{1}{2}\sum_{i<j} \hat{\boldsymbol{e}}_i \cdot \boldsymbol{J}_{i,j} \cdot \hat{\boldsymbol{e}}_j$

where double counting is present (only $$ij$$ is in the sum). Spin vectors are normalized.

• SpinW

(True, False, 1)

$H = \sum_{i,j} \hat{\boldsymbol{S}}_i \cdot \boldsymbol{J}_{i,j} \cdot \hat{\boldsymbol{S}}_j$

where double counting is present ($$ij$$ and $$ji$$ are in the sum). Spin vectors are not normalized.

See Examples for the usage.