diff --git a/src/scgenerator/data/materials.toml b/src/scgenerator/data/materials.toml
index f79930d..08478db 100644
--- a/src/scgenerator/data/materials.toml
+++ b/src/scgenerator/data/materials.toml
@@ -43,6 +43,7 @@ reference = ["CDGM Zemax catalog 2017-09 (obtained from http://www.cdgmgd.com)"]
 [air]
 a = 0.1358
 b = 3.64e-5
+atomic_mass = 4.809e-26
 
 [air.sellmeier]
 B = [57921050000.0, 1679170000.0]
@@ -60,6 +61,7 @@ source = "Pigeon, J. J., Tochitsky, S. Y., Welch, E. C., & Joshi, C. (2016). Mea
 [nitrogen]
 a = 0.137
 b = 1.709e-5
+atomic_mass = 2.3259e-26
 
 [nitrogen.sellmeier]
 B = [32431570000.0]
@@ -80,6 +82,7 @@ a = 0.00346
 b = 2.38e-5
 ionization_energy = 3.9393356074281e-18
 atomic_number = 2
+atomic_mass = 6.646477e-27
 
 [helium.sellmeier]
 B = [2.16463842e-5, 2.10561127e-7, 4.7509272e-5]
@@ -115,6 +118,7 @@ n2 = 3.1e-25
 [hydrogen]
 a = 0.02453
 b = 2.651e-5
+atomic_mass = 1.674e-27
 
 [hydrogen.sellmeier]
 B = [0.0148956, 0.0049037]
@@ -135,6 +139,7 @@ a = 0.02135
 b = 1.709e-5
 ionization_energy = 3.45501365359425e-18
 atomic_number = 10
+atomic_mass = 3.35092e-26
 
 [neon.sellmeier]
 B = [1281450000.0, 22048600000.0]
@@ -154,6 +159,7 @@ a = 0.1355
 b = 3.201e-5
 ionization_energy = 2.5249661793774e-18
 atomic_number = 18
+atomic_mass = 6.6338e-26
 
 [argon.sellmeier]
 B = [2501410000.0, 500283000.0, 52234300000.0]
@@ -211,6 +217,7 @@ a = 0.2349
 b = 3.978e-5
 ionization_energy = 2.2429831039374e-18
 atomic_number = 36
+atomic_mass = 1.3915e-25
 
 [krypton.sellmeier]
 B = [2536370000.0, 2736490000.0, 62080200000.0]
@@ -230,6 +237,7 @@ a = 0.425
 b = 5.105e-5
 ionization_energy = 1.94342415157935
 atomic_number = 54
+atomic_mass = 2.18017e-25
 
 [xenon.sellmeier]
 B = [3228690000.0, 3553930000.0, 60676400000.0]
@@ -247,6 +255,7 @@ source = "Wahlstrand, J. K., Cheng, Y. H., & Milchberg, H. M. (2012). High field
 [vacuum]
 a = 0
 b = 0
+atomic_mass = 0
 
 [vacuum.sellmeier]
 B = []
diff --git a/src/scgenerator/physics/materials.py b/src/scgenerator/physics/materials.py
index 8d8e073..c8cd186 100644
--- a/src/scgenerator/physics/materials.py
+++ b/src/scgenerator/physics/materials.py
@@ -2,6 +2,7 @@ import functools
 from typing import Any
 
 import numpy as np
+from dataclasses import dataclass, field
 
 from .. import utils
 from ..cache import np_cache
@@ -10,6 +11,76 @@ from . import units
 from .units import NA, c, kB, epsilon0
 
 
+@dataclass
+class Sellmeier:
+    B: list[float] = field(default_factory=list)
+    C: list[float] = field(default_factory=list)
+    pressure_ref: float = 101325
+    temperature_ref: float = 273.15
+    kind: int = 2
+    constant: float = 0
+
+    def chi(self, wl: np.ndarray, temperature=None, pressure=None) -> np.ndarray:
+        """n^2 - 1"""
+        chi = np.zeros_like(wl)  # = n^2 - 1
+        if self.kind == 1:
+            for b, c_ in zip(self.B, self.C):
+                chi += wl ** 2 * b / (wl ** 2 - c_)
+        elif self.kind == 2:  # gives n-1
+            for b, c_ in zip(self.B, self.C):
+                chi += b / (c_ - 1 / wl ** 2)
+            chi += self.constant
+            chi = (chi + 1) ** 2 - 1
+        elif self.kind == 3:  # Schott formula
+            for i, b in reversed(list(enumerate(self.B))):
+                chi += b * wl ** (-2 * (i - 1))
+            chi = chi - 1
+        else:
+            raise ValueError(f"kind {self.kind} is not recognized.")
+
+        if temperature is not None:
+            chi *= self.temperature_ref / temperature
+
+        if pressure is not None:
+            chi *= pressure / self.pressure_ref
+        return chi
+
+
+class GasInfo:
+    sellmeier: Sellmeier
+    n2: float
+    atomic_number: int
+    atomic_mass: float
+
+    def __init__(self, gas_name: str):
+        self.mat_dico = utils.load_material_dico(gas_name)
+        self.n2 = self.mat_dico["kerr"]["n2"]
+        self.atomic_mass = self.mat_dico["atomic_mass"]
+        self.atomic_number = self.mat_dico["atomic_number"]
+
+        s = self.mat_dico.get("sellmeier", {})
+        self.sellmeier = Sellmeier(
+            **{
+                newk: s.get(k, None)
+                for newk, k in zip(
+                    ["B", "C", "pressure_ref", "temperature_ref", "kind", "constant"],
+                    ["B", "C", "P0", "T0", "kind", "const"],
+                )
+                if k in s
+            }
+        )
+
+    def pressure_from_density(self, density: float, temperature: float = None) -> float:
+        temperature = temperature or self.sellmeier.temperature_ref
+        return kB * temperature * density / self.atomic_mass
+
+    def get(self, key, default=None):
+        return self.mat_dico.get(key, default)
+
+    def __getitem__(self, key):
+        return self.mat_dico[key]
+
+
 @np_cache
 def n_gas_2(
     wl_for_disp: np.ndarray, gas_name: str, pressure: float, temperature: float, ideal_gas: bool
@@ -82,7 +153,7 @@ def number_density_van_der_waals(
         b = material_dico.get("b", 0) if b is None else b
         pressure = material_dico["sellmeier"].get("P0", 101325) if pressure is None else pressure
         temperature = (
-            material_dico["sellmeier"].get("t0", 273.15) if temperature is None else temperature
+            material_dico["sellmeier"].get("T0", 273.15) if temperature is None else temperature
         )
     else:
         a = 0 if a is None else a