diff --git a/src/workflow.py b/src/workflow.py
index 9483f57..155105a 100644
--- a/src/workflow.py
+++ b/src/workflow.py
@@ -239,4 +239,75 @@ class Workflow:
                             threshold = params[0]
                             target_min, target_max = params[1], params[2]
                             step_size = 0.001
+                            max_attempts = 15
+                            success = False
+
+                            self.logger.info(f"Targeting {target_min}-{target_max} structures. Initial thr={threshold}")
+
+                            for attempt in range(max_attempts):
+                                self.logger.info(f"--- Attempt {attempt + 1}: Threshold {threshold:.4f} ---")
+
+                                # 构造输入: 203 -> filenames -> 1 (distance) -> threshold
+                                # 格式: "203\ndump.xyz train.xyz nep.txt\n1\n0.01"
+                                input_str = f"203\ndump.xyz train.xyz nep.txt\n1\n{threshold}"
+
+                                # 运行 gpumdkit (输出重定向到 append 模式的日志)
+                                if not run_cmd_with_log(kit_path, step_dir, "select_exec.log", input_str=input_str):
+                                    self.logger.error("gpumdkit execution failed.")
+                                    break  # 致命错误直接退出
+
+                                # 检查 selected.xyz 数量
+                                if not os.path.exists(os.path.join(step_dir, "selected.xyz")):
+                                    self.logger.warning("selected.xyz not found. Retrying...")
+                                    continue
+
+                                try:
+                                    # grep -c "Lat" selected.xyz
+                                    count_out = subprocess.check_output('grep -c "Lat" selected.xyz', shell=True,
+                                                                        cwd=step_dir)
+                                    count = int(count_out.decode().strip())
+                                except:
+                                    count = 0
+
+                                self.logger.info(f"Selected count: {count}")
+
+                                # 判断逻辑
+                                if target_min <= count <= target_max:
+                                    self.logger.info(f"Success! {count} structures selected.")
+                                    success = True
+                                    break
+                                elif count < target_min:
+                                    # 选少了 -> 阈值太严 -> 降低阈值? (通常距离阈值越低，选的越多？)
+                                    # 假设：Threshold 代表“最小允许距离”。距离 > Thr 才选。
+                                    # 那么 Thr 越小，条件越松，选的越多。
+                                    self.logger.info("Too few. Decreasing threshold (Loosening).")
+                                    threshold -= step_size
+                                    if threshold < 0: threshold = 0.0001
+                                else:  # count > target_max
+                                    # 选多了 -> 提高阈值
+                                    self.logger.info("Too many. Increasing threshold (Tightening).")
+                                    threshold += step_size
+
+                            if success:
+                                self.tracker.mark_done(task_id_select)
+                            else:
+                                self.logger.warning(
+                                    "Failed to reach target count within max attempts. Using last result.")
+                                # 这里可以选择是否标记完成，或者报错暂停。目前标记完成以便继续。
+                                self.tracker.mark_done(task_id_select)
+
+                        # === 分支 B: 按个数/随机筛选 (One-shot) ===
+                        elif method == "random":
+                            min_n, max_n = params[1], params[2]
+                            # 构造输入: 203 -> filenames -> 2 (number) -> min max
+                            input_str = f"203\ndump.xyz train.xyz nep.txt\n2\n{min_n} {max_n}"
+
+                            self.logger.info(f"Random selection: {min_n}-{max_n}")
+                            if run_cmd_with_log(kit_path, step_dir, "select_exec.log", input_str=input_str):
+                                self.tracker.mark_done(task_id_select)
+                            else:
+                                self.logger.error("Random selection failed.")
+                                return
+
+                    else:
                         self.logger.info("Skipping Select (Already Done).")
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