Python 绘制一个众所周知的双井表面

I am trying to plot a well known energy landscape from physics, the Muller Brown potential. Taken from the literature (https://arxiv.org/abs/1701.01241):

然而，用我的等高线图，我看不到这两个井，它看起来就像是一个单一的高斯.

import numpy as np
import matplotlib.pyplot as plt
import matplotlib as mp

# Parameters of Muller Brown Potential (cf. Bonfati & Cob 2017)
A = np.array([-200, -100, -170, 15])
a = np.array([-1, -1, -6.5, 0.7])
b = np.array([0, 0, 11, 0.6])
c = np.array([-10, -10, -6.5, 0.7])
x_m = np.array([1, 0, -0.5, -1])
y_m = np.array([0, 0.5, 1.5, 1])

x = np.linspace(-1.5, 1, 1000)
y = np.linspace(-0.5, 2, 1000)
XX, YY = np.meshgrid(x, y)


Z =A[0]*np.exp( a[0]*(XX-x_m[0])**2 + b[0]*(XX-x_m[0])*(YY-y_m[0]) + c[0]*(YY-y_m[0])**2 )
+  A[1]*np.exp( a[1]*(XX-x_m[1])**2 + b[1]*(XX-x_m[1])*(YY-y_m[1]) + c[1]*(YY-y_m[1])**2 )
+  A[2]*np.exp( a[2]*(XX-x_m[2])**2 + b[2]*(XX-x_m[2])*(YY-y_m[2]) + c[2]*(YY-y_m[2])**2 )
+  A[3]*np.exp( a[3]*(XX-x_m[3])**2 + b[3]*(XX-x_m[3])*(YY-y_m[3]) + c[3]*(YY-y_m[3])**2 )



fig, ax = plt.subplots()

c=ax.contourf(XX, YY, Z)
plt.colorbar(c)
ax.set_xlabel('x')
ax.set_ylabel('y')

edit:将绘图区域的界限设置为作者似乎没有帮助的界限.

x = np.linspace(-2, 0, 1000)
y = np.linspace(0, 2, 1000)

I see this: