From aefe9bc1b6eebc232ccb188c33ff5c6214ddf1db Mon Sep 17 00:00:00 2001
From: Bob Myhill <myhill.bob@gmail.com>
Date: Mon, 25 Nov 2024 10:45:36 +0000
Subject: [PATCH] fix BM4, MT pressure calcs

---
 burnman/eos/birch_murnaghan_4th.py | 11 +++++++----
 burnman/eos/modified_tait.py       |  2 +-
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/burnman/eos/birch_murnaghan_4th.py b/burnman/eos/birch_murnaghan_4th.py
index 93ee5b4a..52815b28 100644
--- a/burnman/eos/birch_murnaghan_4th.py
+++ b/burnman/eos/birch_murnaghan_4th.py
@@ -45,14 +45,17 @@ def bulk_modulus_fourth(volume, params):
 
 
 def volume_fourth_order(pressure, params):
-    func = lambda x: birch_murnaghan_fourth(params["V_0"] / x, params) - pressure
+
+    def delta_pressure(x):
+        return birch_murnaghan_fourth(params["V_0"] / x, params) - pressure
+
     try:
-        sol = bracket(func, params["V_0"], 1.0e-2 * params["V_0"])
+        sol = bracket(delta_pressure, params["V_0"], 1.0e-2 * params["V_0"])
     except:
         raise ValueError(
             "Cannot find a volume, perhaps you are outside of the range of validity for the equation of state?"
         )
-    return opt.brentq(func, sol[0], sol[1])
+    return opt.brentq(delta_pressure, sol[0], sol[1])
 
 
 def birch_murnaghan_fourth(x, params):
@@ -93,7 +96,7 @@ def volume(self, pressure, temperature, params):
         return volume_fourth_order(pressure, params)
 
     def pressure(self, temperature, volume, params):
-        return birch_murnaghan_fourth(volume / params["V_0"], params)
+        return birch_murnaghan_fourth(params["V_0"] / volume, params)
 
     def isothermal_bulk_modulus_reuss(self, pressure, temperature, volume, params):
         """
diff --git a/burnman/eos/modified_tait.py b/burnman/eos/modified_tait.py
index 90d176b2..f59d7399 100644
--- a/burnman/eos/modified_tait.py
+++ b/burnman/eos/modified_tait.py
@@ -113,7 +113,7 @@ def pressure(self, temperature, volume, params):
         """
         Returns pressure [Pa] as a function of temperature [K] and volume[m^3]
         """
-        return modified_tait(params["V_0"] / volume, params)
+        return modified_tait(volume / params["V_0"], params)
 
     def isothermal_bulk_modulus_reuss(self, pressure, temperature, volume, params):
         """