Accurate determination of the full momentum-dependent spin susceptibility $\chi (\mathbf{q})$ is very important for the description of magnetism and superconductivity. While in principle the formalism for calculating $\chi (\mathbf{q})$ in the linear response density functional theory (DFT) is well established, hardly any publicly available code includes this capability. Here, we describe an alternative way to calculate the static $\chi (\mathbf{q})$, which can be applied to most common DFT codes without additional programming. The method combines standard fixed-spin-moment calculations of $\chi (\mathbf{0})$ with direct calculations of the energy of spin spirals stabilized by an artificial Hubbard interaction. From these calculations, ${\chi }_{\mathit{DFT}}(\mathbf{q})$ can be extracted by inverting the RPA formula. We apply this recipe to the recently discovered Ising superconductivity in NbSe₂ monolayer, one of the most exciting findings in superconductivity in recent years. It was proposed that spin fluctuations may strongly affect the parity of the order parameter. Previous estimates suggested proximity to ferromagnetism, $i.e$., $\chi (\mathbf{q})$ peaked at $\mathbf{q}=0$. We find that the structure of spin fluctuations is more complicated, with the fluctuation spectrum sharply peaked at $\mathbf{q}\approx (0.2,0)$. Such a spectrum would change the interband pairing interaction and considerably affect the superconducting state.

准确确定完整的动量相关自旋磁化率 $\chi (\mathbf{q})$对于描述磁性和超导性非常重要。虽然原则上计算的形式主义$\chi (\mathbf{q})$在线性响应密度泛函理论 (DFT) 中，几乎没有任何公开可用的代码包含此功能。在这里，我们描述了一种计算静态的替代方法$\chi (\mathbf{q})$，无需额外编程即可应用于最常见的 DFT 代码。该方法结合了标准的固定自旋力矩计算$\chi (\mathbf{0})$直接计算由人工哈伯德相互作用稳定的自旋螺旋的能量。根据这些计算，${\chi }_{\mathit{离散傅立叶变换}}(\mathbf{q})$可以通过反转 RPA 公式来提取。我们将此配方应用于最近发现的 NbSe ₂单层中的Ising 超导性，这是近年来超导性中最令人兴奋的发现之一。有人提出自旋波动可能强烈影响阶参数的奇偶性。先前的估计表明接近铁磁性，$我.\mathrm{电子}$., $\chi (\mathbf{q})$ 达到顶峰 $\mathbf{q}=0$. 我们发现自旋涨落的结构更加复杂，涨落谱在$\mathbf{q}\approx (0.2,0)$. 这样的光谱会改变带间配对相互作用并显着影响超导状态。