Dear Sir,

```
I have calculated phonon dispersion of SnS monolayer using 5x5x1 supercell with K mesh 5x5x1. I am getting negative frequencies around Gamma point. To remove the error I tried to apply the rotational sum rules for this structure. I have SPOSCAR, FORCE_CONSTANTS obtained from Phonopy, POSCAR files. I am using using the following script to obtain ForceConstantPotential:
```

…

**from ase.io import read**

**from hiphive import ClusterSpace, ForceConstantPotential**

**from hiphive.cutoffs import estimate_maximum_cutoff**

**from hiphive.utilities import extract_parameters**

**from hiphive import ForceConstants, ClusterSpace, ForceConstantPotential**

**from hiphive import enforce_rotational_sum_rules**

**from hiphive.utilities import extract_parameters**

**prim = read(‘POSCAR’)**

**supercell = read(‘SPOSCAR’)**

**# Define a cluster space using the largest cutoff you can**

**max_cutoff = estimate_maximum_cutoff(supercell) - 0.01**

**cutoffs = [8.0] # only second order needed**

**cs = ClusterSpace(prim, cutoffs)**

**# import the phonopy force constants using the correct supercell also provided by phonopy**

**fcs =ForceConstants.read_phonopy(supercell,‘FORCE_CONSTANTS’)**

**# Find the parameters that best fits the force constants given you cluster space**

**parameters = extract_parameters(fcs, cs)**

**fcp = ForceConstantPotential(fcs, parameters)**

…

The error I am getting is as follows:

…

**Primitive cell:**

** Formula: S2Sn2**

** Cell:**

** [ 4.16278 0.00000 0.00000]**

** [ 0.00000 4.36945 0.00000]**

** [ 0.00000 0.00000 20.45051]**

** Basis:**

** Sn [ 0.25000 0.38112 0.36012]**

** Sn [ 0.75000 0.88112 0.49733]**

** S [ 0.75000 0.97299 0.37231]**

** S [ 0.25000 0.47299 0.48514]**

**Crystal symmetry:**

** Spacegroup: Pmn2_1 (31)**

** Unique site: 2**

** Symmetry operations: 4**

** symprec: 1.00e-05**

**Cutoffs:**

** Maximum cutoff: 8.0**

** Found 4 center atoms with 48 images totaling 52 atoms**

**Clusters:**

** Clusters: {2: 162}**

** Total number of clusters: 162**

**Orbits:**

** Orbits: {2: 26}**

** Total number of orbits: 26**

**Eigentensors:**

** Eigentensors: {2: 198}**

** Total number of parameters: 198**

**Constraints:**

** Acoustic: True**

** Number of degrees of freedom: {2: 188}**

** Total number of degrees of freedom: 188**

**Force constant reconstruction error order 2: 6.8425%**

**Traceback (most recent call last):**

** File “/home/safikul_akd/Anaconda_physics/lib/python3.9/site-packages/hiphive/safikul.py”, line 23, in **

** fcp = ForceConstantPotential(fcs, parameters)**

** File “/home/safikul_akd/Anaconda_physics/envs/phonopy/lib/python3.10/site-packages/hiphive/force_constant_potential.py”, line 44, in **init****

** self._prim = cs.primitive_structure.copy()**

**AttributeError: ‘SortedForceConstants’ object has no attribute 'primitive_structure’**

…

Sir, could you please help me by providing the full script to generate ForceConstantPotential and to apply Born-Huang rotational sum rules?

Kind Regards,

Safikul