我想拟合一个这样的函数：

然后出现如图的错误

这是代码：
data = {{0.173, 21.277}, {0.18, 18.663}, {0.187, 12.442}, {0.194,
7.912}, {0.201, 4.591}, {0.208, 2.408}, {0.215,
0.735}, {0.222, -0.38}, {0.229, -1.102}, {0.236, -1.608}, {0.243, -2.051}, {0.249, -2.374}, {0.256, -2.633}, {0.263, -3.042}, {0.27, -3.097}, {0.277, -3.445}, {0.284, -3.605}, {0.291, -3.722}, {0.298, -3.809}, {0.305, -3.875}, {0.312, -3.922}, {0.319, -3.95}, {0.326, -3.961}, {0.333, -3.957}, {0.34, -3.941}, {0.346, -3.912}, {0.353, -4.045}, {0.36, -3.939}, {0.367, -3.925}, {0.374, -3.82}, {0.381, -3.782}, {0.388, -3.675}, {0.395, -3.597}, {0.402, -3.516}, {0.409, -3.432}, {0.416, -3.349}, {0.43, -3.177}, {0.443, -2.992}, {0.457, -2.809}, {0.471, -2.633}, {0.485, -2.459}, {0.499, -2.305}, {0.513, -2.319}, {0.527, -2.064}, {0.54, -2.13}, {0.554, -1.914}, {0.568, -1.854}, {0.582, -1.909}, {0.596, -1.965}, {0.61, -1.888}, {0.624, -1.7}, {0.637, -1.844}, {0.651, -1.744}, {0.665, -1.604}, {0.679, -1.583}, {0.693, -1.674}, {0.707, -1.546}, {0.721, -1.646}, {0.734, -1.516}, {0.748, -1.605}, {0.762, -1.607}, {0.776, -1.597}, {0.79, -1.573}, {0.804, -1.467}};
morse = NonlinearModelFit[data, D0[Exp[-2 a[x - r0]] - 2 Exp[-a[x - r0]]], {D0, a, r0}, x]

D0[...]改D0(...)