运行arch y x, arch(1) garch(1)时显示：

flat log likelihood encountered, cannot find uphill direction

请教是什么原因？

完整信息如下：

. gen date_f = _n

.
. tsset date_f
        time variable:  date_f, 1 to 120

. arch r, arch(1) garch(1)

(setting optimization to BHHH)
Iteration 0:   log likelihood =  264.17693 
Iteration 1:   log likelihood =  264.37667 
Iteration 2:   log likelihood =   264.9259 
Iteration 3:   log likelihood =  265.51798 
Iteration 4:   log likelihood =   265.8754 
(switching optimization to BFGS)
Iteration 5:   log likelihood =  267.14262 
Iteration 6:   log likelihood =   267.1711 
Iteration 7:   log likelihood =  267.19927 
Iteration 8:   log likelihood =  267.40472 
Iteration 9:   log likelihood =  267.46875 
Iteration 10:  log likelihood =  267.50234 
Iteration 11:  log likelihood =  267.53322 
Iteration 12:  log likelihood =  267.61433 
Iteration 13:  log likelihood =  267.65933 
Iteration 14:  log likelihood =  267.78939 
(switching optimization to BHHH)
Iteration 15:  log likelihood =  267.90332 
Iteration 16:  log likelihood =  267.91476  (backed up)
Iteration 17:  log likelihood =  267.93768  (backed up)
Iteration 18:  log likelihood =  267.96717  (backed up)
Iteration 19:  log likelihood =   268.0004  (backed up)
(switching optimization to BFGS)
Iteration 20:  log likelihood =  268.03535  (backed up)
Iteration 21:  log likelihood =  268.10499  (backed up)
Iteration 22:  log likelihood =  268.25464 
Iteration 23:  log likelihood =  268.26533  (backed up)
Iteration 24:  log likelihood =  268.26533  (backed up)
Iteration 25:  log likelihood =  268.26577  (backed up)
Iteration 26:  log likelihood =  268.26577  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (0)
Iteration 27:  log likelihood =  268.26577  (backed up)
Iteration 28:  log likelihood =  268.27276  (backed up)
Iteration 29:  log likelihood =  269.17034  (backed up)
(switching optimization to BHHH)
Iteration 30:  log likelihood =  269.17056  (backed up)
Iteration 31:  log likelihood =  269.17085  (backed up)
Iteration 32:  log likelihood =  269.17088  (backed up)
Iteration 33:  log likelihood =  269.17096  (backed up)
Iteration 34:  log likelihood =  269.17101  (backed up)
(switching optimization to BFGS)
Iteration 35:  log likelihood =  269.17103  (backed up)
Iteration 36:  log likelihood =  269.17237  (backed up)
Iteration 37:  log likelihood =  269.47484 
Iteration 38:  log likelihood =  269.49255  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (1)
Iteration 39:  log likelihood =  269.57863 
Iteration 40:  log likelihood =  270.77836  (backed up)
Iteration 41:  log likelihood =  270.77839  (backed up)
Iteration 42:  log likelihood =  270.79197  (backed up)
Iteration 43:  log likelihood =  270.79197  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (2)
Iteration 44:  log likelihood =    271.019 
(switching optimization to BHHH)
Iteration 45:  log likelihood =    271.019  (backed up)
Iteration 46:  log likelihood =    271.019  (backed up)
Iteration 47:  log likelihood =    271.019  (backed up)
Iteration 48:  log likelihood =    271.019  (backed up)
Iteration 49:  log likelihood =    271.019  (backed up)
(switching optimization to BFGS)
Iteration 50:  log likelihood =    271.019  (backed up)
Iteration 51:  log likelihood =  271.01902  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (3)
Iteration 52:  log likelihood =  271.09784  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (4)
Iteration 53:  log likelihood =  271.09784  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (5)
Iteration 54:  log likelihood =  271.09784  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (6)
Iteration 55:  log likelihood =  271.09784  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (7)
Iteration 56:  log likelihood =  271.09784  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (8)
Iteration 57:  log likelihood =  271.09784  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (9)
Iteration 58:  log likelihood =  271.09784  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (10)
Iteration 59:  log likelihood =  271.09784  (backed up)
(switching optimization to BHHH)
Iteration 60:  log likelihood =  271.09784  (not concave)
Iteration 61:  log likelihood =  271.10392  (not concave)
Iteration 62:  log likelihood =  271.10413 
Iteration 63:  log likelihood =  271.10413  (backed up)
Iteration 64:  log likelihood =  271.10413  (backed up)
(switching optimization to BFGS)
Iteration 65:  log likelihood =  271.10413  (backed up)
Iteration 66:  log likelihood =   271.1043  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (11)
Iteration 67:  log likelihood =  271.17126  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (12)
Iteration 68:  log likelihood =  271.17126  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (13)
Iteration 69:  log likelihood =  271.17126  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (14)
Iteration 70:  log likelihood =  271.17126  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (15)
Iteration 71:  log likelihood =  271.17126  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (16)
Iteration 72:  log likelihood =  271.17126  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (17)
Iteration 73:  log likelihood =  271.17126  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (18)
Iteration 74:  log likelihood =  271.17126  (backed up)
(switching optimization to BHHH)
Iteration 75:  log likelihood =  271.17126  (not concave)
Iteration 76:  log likelihood =   271.1742 
Iteration 77:  log likelihood =   271.1742  (backed up)
Iteration 78:  log likelihood =   271.1742  (backed up)
Iteration 79:  log likelihood =   271.1742  (backed up)
(switching optimization to BFGS)
Iteration 80:  log likelihood =   271.1742  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (19)
Iteration 81:  log likelihood =  271.17459  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (20)
Iteration 82:  log likelihood =  271.17459  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (21)
Iteration 83:  log likelihood =  271.17459  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (22)
Iteration 84:  log likelihood =  271.17459  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (23)
Iteration 85:  log likelihood =  271.17459  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (24)
Iteration 86:  log likelihood =  271.17459  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (25)
Iteration 87:  log likelihood =  271.17459  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (26)
Iteration 88:  log likelihood =  271.17459  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (27)
Iteration 89:  log likelihood =  271.17459  (backed up)
(switching optimization to BHHH)
Iteration 90:  log likelihood =  271.17459  (backed up)
Iteration 91:  log likelihood =  271.17459  (backed up)
Iteration 92:  log likelihood =  271.17459  (backed up)
Iteration 93:  log likelihood =  271.17459  (backed up)
Iteration 94:  log likelihood =  271.17459  (backed up)
(switching optimization to BFGS)
Iteration 95:  log likelihood =  271.17459  (backed up)
Iteration 96:  log likelihood =  271.17471  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (28)
Iteration 97:  log likelihood =  271.24167  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (29)
Iteration 98:  log likelihood =  271.24167  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (30)
Iteration 99:  log likelihood =  271.24167  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (31)
Iteration 100: log likelihood =  271.24167  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (32)
Iteration 101: log likelihood =  271.24167  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (33)
Iteration 102: log likelihood =  271.24167  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (34)
Iteration 103: log likelihood =  271.24167  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (35)
Iteration 104: log likelihood =  271.24167  (backed up)
(switching optimization to BHHH)
Iteration 105: log likelihood =  271.24167  (not concave)
Iteration 106: log likelihood =  271.24742 
Iteration 107: log likelihood =  271.24742  (backed up)
Iteration 108: log likelihood =  271.24742  (backed up)
Iteration 109: log likelihood =  271.24742  (not concave)
(switching optimization to BFGS)
Iteration 110: log likelihood =  271.24752 
Iteration 111: log likelihood =  271.66191  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (36)
Iteration 112: log likelihood =  271.91856  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (37)
Iteration 113: log likelihood =  271.91856  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (38)
Iteration 114: log likelihood =  271.91856  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (39)
Iteration 115: log likelihood =  271.91856  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (40)
Iteration 116: log likelihood =  271.91856  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (41)
Iteration 117: log likelihood =  271.91856  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (42)
Iteration 118: log likelihood =  271.91856  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (43)
Iteration 119: log likelihood =  271.91856  (backed up)
(switching optimization to BHHH)
Iteration 120: log likelihood =  271.91856  (not concave)
Iteration 121: log likelihood =  271.93553  (not concave)
Iteration 122: log likelihood =  271.93769  (not concave)
Iteration 123: log likelihood =  271.93779 
Iteration 124: log likelihood =  271.93779  (backed up)
(switching optimization to BFGS)
Iteration 125: log likelihood =  271.93779  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (44)
Iteration 126: log likelihood =  271.94214  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (45)
Iteration 127: log likelihood =  271.94214  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (46)
Iteration 128: log likelihood =  271.94214  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (47)
Iteration 129: log likelihood =  271.94214  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (48)
Iteration 130: log likelihood =  271.94214  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (49)
Iteration 131: log likelihood =  271.94214  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (50)
Iteration 132: log likelihood =  271.94214  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (51)
Iteration 133: log likelihood =  271.94214  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (52)
Iteration 134: log likelihood =  271.94214  (backed up)
(switching optimization to BHHH)
Iteration 135: log likelihood =  271.94214  (not concave)
Iteration 136: log likelihood =  271.94507 
Iteration 137: log likelihood =  271.94507  (backed up)
Iteration 138: log likelihood =  271.94507  (backed up)
Iteration 139: log likelihood =  271.94507  (backed up)
(switching optimization to BFGS)
Iteration 140: log likelihood =  271.94507  (backed up)
Iteration 141: log likelihood =  271.94516  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (53)
Iteration 142: log likelihood =  272.03187 
BFGS stepping has contracted, resetting BFGS Hessian (54)
Iteration 143: log likelihood =  272.03187  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (55)
Iteration 144: log likelihood =  272.03187  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (56)
Iteration 145: log likelihood =  272.03187  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (57)
Iteration 146: log likelihood =  272.03187  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (58)
Iteration 147: log likelihood =  272.03187  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (59)
Iteration 148: log likelihood =  272.03187  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (60)
Iteration 149: log likelihood =  272.03187  (backed up)
(switching optimization to BHHH)
Iteration 150: log likelihood =  272.03187  (not concave)
Iteration 151: log likelihood =  272.03947  (not concave)
Iteration 152: log likelihood =  272.03994  (not concave)
Iteration 153: log likelihood =  272.03995 
Iteration 154: log likelihood =  272.03995  (backed up)
(switching optimization to BFGS)
Iteration 155: log likelihood =  272.03995  (backed up)
Iteration 156: log likelihood =  272.04708  (backed up)
Iteration 157: log likelihood =  272.15075 
BFGS stepping has contracted, resetting BFGS Hessian (61)
Iteration 158: log likelihood =  273.32224  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (62)
Iteration 159: log likelihood =  273.32224  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (63)
Iteration 160: log likelihood =  273.32224  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (64)
Iteration 161: log likelihood =  273.32224  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (65)
Iteration 162: log likelihood =  273.32224  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (66)
Iteration 163: log likelihood =  273.32224  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (67)
Iteration 164: log likelihood =  273.32224  (backed up)
(switching optimization to BHHH)
Iteration 165: log likelihood =  273.32224  (not concave)
Iteration 166: log likelihood =   273.5091  (not concave)
Iteration 167: log likelihood =  273.52316  (not concave)
Iteration 168: log likelihood =  273.52492  (not concave)
Iteration 169: log likelihood =  273.52498  (not concave)
(switching optimization to BFGS)
Iteration 170: log likelihood =  273.52499 
Iteration 171: log likelihood =  273.52499 
Iteration 172: log likelihood =  273.52499 
BFGS stepping has contracted, resetting BFGS Hessian (68)
Iteration 173: log likelihood =   273.5251 
BFGS stepping has contracted, resetting BFGS Hessian (69)
Iteration 174: log likelihood =   273.5251  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (70)
Iteration 175: log likelihood =   273.5251  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (71)
Iteration 176: log likelihood =   273.5251  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (72)
Iteration 177: log likelihood =   273.5251  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (73)
Iteration 178: log likelihood =   273.5251  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (74)
Iteration 179: log likelihood =   273.5251  (backed up)
(switching optimization to BHHH)
Iteration 180: log likelihood =   273.5251  (not concave)
Iteration 181: log likelihood =   273.5252  (not concave)
Iteration 182: log likelihood =   273.5252  (not concave)
Iteration 183: log likelihood =   273.5252  (not concave)
Iteration 184: log likelihood =   273.5252 
(switching optimization to BFGS)
Iteration 185: log likelihood =   273.5252  (backed up)
Iteration 186: log likelihood =   273.5252  (backed up)
Iteration 187: log likelihood =  273.52521  (backed up)
Iteration 188: log likelihood =  273.61754  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (75)
Iteration 189: log likelihood =  273.94084 
BFGS stepping has contracted, resetting BFGS Hessian (76)
Iteration 190: log likelihood =  273.94084  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (77)
Iteration 191: log likelihood =  273.94084  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (78)
Iteration 192: log likelihood =  273.94084  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (79)
Iteration 193: log likelihood =  273.94084  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (80)
Iteration 194: log likelihood =  273.94084  (backed up)
(switching optimization to BHHH)
Iteration 195: log likelihood =  273.94084  (not concave)
Iteration 196: log likelihood =  274.56026  (not concave)
Iteration 197: log likelihood =   274.7681  (not concave)
Iteration 198: log likelihood =  274.78119  (not concave)
Iteration 199: log likelihood =  274.78196  (not concave)
(switching optimization to BFGS)
Iteration 200: log likelihood =  274.78196 
Iteration 201: log likelihood =  274.78197 
Iteration 202: log likelihood =  274.78197 
Iteration 203: log likelihood =  274.78197 
Iteration 204: log likelihood =  274.78197 
Iteration 205: log likelihood =  274.78197 
BFGS stepping has contracted, resetting BFGS Hessian (81)
Iteration 206: log likelihood =  274.78199 
BFGS stepping has contracted, resetting BFGS Hessian (82)
Iteration 207: log likelihood =  274.78199  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (83)
Iteration 208: log likelihood =  274.78199  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (84)
Iteration 209: log likelihood =  274.78199  (backed up)
(switching optimization to BHHH)
Iteration 210: log likelihood =  274.78199  (not concave)
Iteration 211: log likelihood =  274.78202  (not concave)
Iteration 212: log likelihood =  274.78202  (not concave)
Iteration 213: log likelihood =  274.78202  (not concave)
Iteration 214: log likelihood =  274.78202  (not concave)
(switching optimization to BFGS)
Iteration 215: log likelihood =  274.78202 
Iteration 216: log likelihood =  274.78289  (backed up)
Iteration 217: log likelihood =  274.78757  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (85)
Iteration 218: log likelihood =  274.78757  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (86)
Iteration 219: log likelihood =  274.78757  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (87)
Iteration 220: log likelihood =  274.78757  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (88)
Iteration 221: log likelihood =  274.78757  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (89)
Iteration 222: log likelihood =  274.78757  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (90)
Iteration 223: log likelihood =  274.78757  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (91)
Iteration 224: log likelihood =  274.78757  (backed up)
(switching optimization to BHHH)
Iteration 225: log likelihood =  274.78757  (not concave)
Iteration 226: log likelihood =  274.78918  (not concave)
Iteration 227: log likelihood =  274.78924  (not concave)
Iteration 228: log likelihood =  274.78925  (not concave)
Iteration 229: log likelihood =  274.78925  (not concave)
(switching optimization to BFGS)
Iteration 230: log likelihood =  274.78925 
Iteration 231: log likelihood =  274.78953  (backed up)
Iteration 232: log likelihood =  274.81791  (backed up)
Iteration 233: log likelihood =  274.81903  (backed up)
Iteration 234: log likelihood =  274.82771  (backed up)
Iteration 235: log likelihood =  274.82771  (backed up)
Iteration 236: log likelihood =  274.82771  (backed up)
Iteration 237: log likelihood =  274.82961  (backed up)
Iteration 238: log likelihood =  274.83076 
Iteration 239: log likelihood =  274.83081 
(switching optimization to BHHH)
Iteration 240: log likelihood =  274.83081  (not concave)
Iteration 241: log likelihood =  274.83081  (not concave)
Iteration 242: log likelihood =  274.83081  (not concave)
Iteration 243: log likelihood =  274.83081  (not concave)
Iteration 244: log likelihood =  274.83081  (not concave)
(switching optimization to BFGS)
Iteration 245: log likelihood =  274.83081 
Iteration 246: log likelihood =  274.83084  (backed up)
Iteration 247: log likelihood =  274.83191  (backed up)
Iteration 248: log likelihood =  274.83352  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (92)
Iteration 249: log likelihood =  274.83556 
BFGS stepping has contracted, resetting BFGS Hessian (93)
Iteration 250: log likelihood =  274.83556  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (94)
Iteration 251: log likelihood =  274.83556  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (95)
Iteration 252: log likelihood =  274.83556  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (96)
Iteration 253: log likelihood =  274.83556  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (97)
Iteration 254: log likelihood =  274.83556  (backed up)
(switching optimization to BHHH)
Iteration 255: log likelihood =  274.83556  (not concave)
Iteration 256: log likelihood =  274.83833  (not concave)
Iteration 257: log likelihood =  274.83834  (not concave)
Iteration 258: log likelihood =  274.83834  (not concave)
Iteration 259: log likelihood =  274.83834  (not concave)
(switching optimization to BFGS)
Iteration 260: log likelihood =  274.83834 
Iteration 261: log likelihood =  274.83863  (backed up)
Iteration 262: log likelihood =  274.83863  (backed up)
Iteration 263: log likelihood =  274.84874  (backed up)
Iteration 264: log likelihood =  274.85435  (backed up)
Iteration 265: log likelihood =  274.85435  (backed up)
Iteration 266: log likelihood =  274.85435  (backed up)
Iteration 267: log likelihood =  274.85435  (backed up)
Iteration 268: log likelihood =  274.85435  (backed up)
BFGS stepping has contracted, resetting BFGS Hessian (98)
Iteration 269: log likelihood =  274.85435  (backed up)
(switching optimization to BHHH)
Iteration 270: log likelihood =  274.85435  (not concave)
Iteration 271: log likelihood =  274.85831  (not concave)
Iteration 272: log likelihood =  274.85877  (not concave)
Iteration 273: log likelihood =  274.85878  (not concave)
Iteration 274: log likelihood =  274.85878  (not concave)
(switching optimization to BFGS)
Iteration 275: log likelihood =  274.85878 
Iteration 276: log likelihood =  274.85878 
Iteration 277: log likelihood =  274.85878 
Iteration 278: log likelihood =  274.85878 
BFGS stepping has contracted, resetting BFGS Hessian (99)
Iteration 279: log likelihood =   274.8591 
flat log likelihood encountered, cannot find uphill direction
r(430);